Two chamber F2 laser system with F2 pressure based line selection

ABSTRACT

The present invention provides an injection seeded modular gas discharge laser system capable of producing high quality pulsed laser beams at pulse rates of about 4,000 Hz or greater and at pulse energies of about 5 to 10 mJ or greater for integrated outputs of about 20 to 40 Watts or greater. Two separate discharge chambers are provided, one of which is a part of a master oscillator producing a very narrow band seed beam which is amplified in the second discharge chamber. The parameters chamber can be controlled separately permitting optimization of wavelength parameters in the master oscillator and optimization of pulse energy parameters in the amplifying chamber. A preferred embodiment is a F 2  laser system configured as a MOPA and specifically designed for use as a light source for integrated circuit lithography. In this preferred embodiment, both of the chambers and the laser optics are mounted on a vertical optical table within a laser enclosure. In the preferred MOPA embodiment, each chamber comprises a single tangential fan providing sufficient gas flow to permit operation at pulse rates of 4000 Hz or greater by clearing debris from the discharge region in less time than the approximately 0.25 milliseconds between pulses. The master oscillator is operated with a fluorine partial pressure and total gas pressure within specified ranges in order to reduce the intensity of the weak line to less than 0.01% of the strong line. Therefore, the need for line selection optical equipment is avoided.

The present invention is a continuation-in-part of Ser. No. 10/233,253,filed Aug. 30, 2002, now U.S. Pat. No. 6,704,339 Ser. No. 10/210,761,filed Jul. 31, 2002, now U.S. Pat. No. 6,690,704, Ser. No. 10/187,336,filed Jun. 28, 2002, Ser. No. 10/141,216, filed May 7, 2002, now U.S.Pat. No. 6,693,939 Ser. No. 10/056,619, filed Jan. 23, 2002, all ofwhich is incorporated by reference herein. This invention relates to F₂lasers and in particular to narrow band F₂ lasers.

BACKGROUND OF THE INVENTION Electric Discharge Gas Lasers

Electric discharge gas lasers are well known and have been availablesince soon after lasers were invented in the 1960s. A high voltagedischarge between two electrodes excites a laser gas to produce agaseous gain medium. A resonance cavity containing the gain mediumpermits stimulated amplification of light which is then extracted fromthe cavity in the form of a laser beam. Many of these electric dischargegas lasers are operated in a pulse mode.

Excimer Lasers

Excimer lasers are a particular type of electric discharge gas laser andthey have been known since the mid 1970s. A description of an excimerlaser, useful for integrated circuit lithography, is described in U.S.Pat. No. 5,023,884 issued Jun. 11, 1991 entitled “Compact ExcimerLaser.” This patent has been assigned to Applicants' employer, and thepatent is hereby incorporated herein by reference. The excimer laserdescribed in Patent '884 is a high repetition rate pulse laser. Theseexcimer lasers, when used for integrated circuit lithography, aretypically operated in a fabrication line “around-the-clock” producingmany thousands of valuable integrated circuits per hour; therefore,down-time can be very expensive. For this reason most of the componentsare organized into modules which can be replaced within a few minutes.Excimer lasers used for lithography typically must have its output beamreduced in bandwidth to a fraction of a picometer. Electric dischargegas lasers of the type described in Patent '884 utilize an electricpulse power system to produce the electrical discharges, between the twoelectrodes. In such prior art systems, a direct current power supplycharges a capacitor bank called “the charging capacitor” or “C₀” to apredetermined and controlled voltage called the “charging voltage” foreach pulse. The magnitude of this charging voltage may be in the rangeof about 500 to 1000 volts in these prior art units. After C₀ has beencharged to the predetermined voltage, a solid state switch is closedallowing the electrical energy stored on C₀ to ring very quickly througha series of magnetic compression circuits and a voltage transformer toproduce high voltage electrical potential in the range of about 16,000volts (or greater) across the electrodes which produce the dischargeswhich lasts about 20 to 50 ns.

Major Advances in Lithography Light Sources

Excimer lasers such as described in the '884 patent have during theperiod 1989 to 2001 become the primary light source for integratedcircuit lithography. More than 1000 of these lasers are currently in usein the most modern integrated circuit fabrication plants. Almost all ofthese lasers have the basic design features described in the '884patent.

This is:

(1) a single, pulse power system for providing electrical pulses acrossthe electrodes at pulse rates of about 100 to 2500 pulses per second;

(2) a single resonant cavity comprised of a partially reflectingmirror-type output coupler and a line narrowing unit consisting of aprism beam expander, a tuning mirror and a grating;

(3) a single discharge chamber containing a laser gas (either KrF orArF), two elongated electrodes and a tangential fan for circulating thelaser gas between the two electrodes fast enough to clear the dischargeregion between pulses, and

(4) a beam monitor for monitoring pulse energy, wavelength and bandwidthof output pulses with a feedback control system for controlling pulseenergy, energy dose and wavelength on a pulse-to-pulse basis.

During the 1989-2001 period, output power of these lasers has increasedgradually and beam quality specifications for pulse energy stability,wavelength stability and bandwidth have also become increasinglytighter. Operating parameters for a popular lithography laser model usedwidely in integrated circuit fabrication include pulse energy at 8 mJ,pulse rate at 2,500 pulses per second (providing an average beam powerof up to about 20 watts), bandwidth at about 0.5 pm (FWHM) and pulseenergy stability at +/−0.35%.

F₂ Lasers

F₂ lasers are well known. These lasers are similar to the KrF and ArFlasers. The basic differences are the gas mixture which in the F₂ laseris a small portion of F₂ with helium and/or neon as a buffer gas. Thenatural output spectrum of the F₂ laser is concentrated in two spectrallines of narrow bandwidth, a relatively strong line centered at about157.63 nm and a relatively weak line centered at about 157.52 nm.

F₂ Lasers Bandwidth

A typical KrF laser has a natural bandwidth of about 300 pm measuredfull width half maximum (FWHM) centered at about 248 nm and forlithography use, it is typically line narrowed to less than 0.6 pm. (Inthis specification bandwidth values will refer to the FWHM bandwidthsunless otherwise indicated.) ArF lasers have a natural bandwidth ofabout 500 centered at about 193 nm and is typically line narrowed toless than 0.5 pm. These lasers can be relatively easily tuned over alarge portion of their natural bandwidth using the grating based linenarrowing module referred to above. F₂ lasers, as stated above,typically produce laser beams with most of its energy in two narrowspectral features (referred to herein sometimes as “spectral lines”)centered at about 157.63 nm and 157.52 nm. Often, the less intense ofthese two spectral lines (i.e., the 157.52 nm line) is suppressed andthe laser is forced to operate at the 157.63 nm line. A known techniquefor suppressing the weak line is to spectrally spread the laser beamusing beam dispersing optics within the resonant cavity of the laser andto spatially block the weak line at an aperture. The natural bandwidthof the 157.63 nm line is pressure and gas content dependent and variesfrom about 0.6 to 1.2 pm (FWHM). An F₂ laser with a bandwidth in thisrange can be used with lithography devices with a catadiophic lensdesign utilizing both refractive and reflective optical elements, butfor an all-refractive lens design the laser beam bandwidth may have tobe reduced to about 0.1 pm to produce desired results.

Injection Seeding

A well-known technique for reducing the band-width of gas dischargelaser systems (including excimer laser systems) involves the injectionof a narrow band “seed” beam into a gain medium. In one such system, alaser producing the seed beam called a “master oscillator” is designedto provide a very narrow bandwidth beam in a first gain medium, and thatbeam is used as a seed beam in a second gain medium. If the second gainmedium functions as a power amplifier, the system is referred to as amaster oscillator, power amplifier (MOPA) system. If the second gainmedium itself has a resonance cavity (in which laser oscillations takeplace), the system is referred to as an injection seeded oscillator(ISO) system or a master oscillator, power oscillator (MOPO) system inwhich case the seed laser is called the master oscillator and thedownstream system is called the power oscillator. Laser systemscomprised of two separate systems tend to be substantially moreexpensive, larger and more complicated than comparable single chamberlaser systems. Therefore, commercial application of these two chamberlaser systems has been limited.

What is needed is a better laser design for a pulse gas discharge F₂laser for operation at repetition rates in the range of about 4,000pulses per second or greater, permitting precise control of all beamquality parameters including wavelength and pulse energy.

SUMMARY OF THE INVENTION

The present invention provides an injection seeded modular gas dischargelaser system capable of producing high quality pulsed laser beams atpulse rates of about 4,000 Hz or greater and at pulse energies of about5 to 10 mJ or greater for integrated outputs of about 20 to 40 Watts orgreater. Two separate discharge chambers are provided, one of which is apart of a master oscillator producing a very narrow band seed beam whichis amplified in the second discharge chamber. The parameters chamber canbe controlled separately permitting optimization of wavelengthparameters in the master oscillator and optimization of pulse energyparameters in the amplifying chamber. A preferred embodiment is a F₂laser system configured as a MOPA and specifically designed for use as alight source for integrated circuit lithography. In this preferredembodiment, both of the chambers and the laser optics are mounted on avertical optical table within a laser enclosure. In the preferred MOPAembodiment, each chamber comprises a single tangential fan providingsufficient gas flow to permit operation at pulse rates of 4000 Hz orgreater by clearing debris from the discharge region in less time thanthe approximately 0.25 milliseconds between pulses. The masteroscillator is operated with a fluorine partial pressure and total gaspressure within specified ranges in order to reduce the intensity of theweak line to less than 0.01% of the strong line. Therefore, the need forline selection optical equipment is avoided. Preferred embodiments mayalso include a pulse multiplying module dividing each pulse from thepower amplifier into two pulses in order to reduce substantially thedeterioration rates of lithography optics. Preferred embodiments of thisinvention utilize a “three wavelength platform”. This includes anenclosure optics table and general equipment layout that is the same foreach of the three types of discharge laser systems expected to be insubstantial use for integrated circuit fabrication during the early partof the 21^(st) century, i.e., KrF, ArF, and F₂ lasers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing of a preferred embodiment of the present invention.

FIG. 1A shows a test set-up.

FIGS. 2 and 3 show chamber features.

FIGS. 4 and 4A through 4E show features of a preferred pulse powersystem.

FIGS. 5A, 5B, 5C1, 5C2, 5C3 and 5D show additional pulse power features.

FIGS. 6, 7, 8, 9A, 9B, 10, 10A, 11, 12, 12A, 12B show features of pulsepower components.

FIGS. 13A1-13A6 show features of a preferred current return structure.

FIGS. 14 and 15 show electrode features.

FIG. 16 shows a fan motor drive arrangement.

FIG. 17 show a preferred fan blade.

FIGS. 18 and 19A through 19G show features of a purge system.

FIGS. 20, 20A and 20B show features of a preferred shutter.

FIGS. 21 and 21A show heat exchanger features.

FIGS. 22A through 22D show features of a pulse multiplier unit.

FIGS. 23, 23A and 23B show techniques for spatially filtering a seedbeam.

FIGS. 24A-24L are graphs of test data.

FIG. 25 is a drawing showing a technique for monitoring the relativeintensity of the fluorine weak line.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS First Preferred EmbodimentFirst General Layout

FIG. 1 is block drawing showing the principle components of a firstpreferred embodiment of the present invention. This embodiment is aninjection seeded narrow band F₂ laser system configured as a MOPA lasersystem. It is specially designed for use as a light source forintegrated circuit lithography. The major improvements in the presentinvention as exemplified in this embodiment over the prior art KrF andArF lithography lasers is the conversion to the F₂ 157.63 nm wavelengthlight source and the utilization of injection seeding. In thisembodiment a master oscillator-power amplifier (MOPA) configuration withtwo separate discharge chambers and control of F₂ gas partial pressureand the total gas pressure in the master oscillator virtually eliminatesthe 157.52 nm weak line from the spectrum of the output beam.

This first preferred embodiment utilizes a modular platformconfiguration which is designed to accommodate either fluorine (F₂),krypton-fluoride (KrF), or argon-fluoride (ArF) laser components. Thisplatform design permits use of the same basic cabinet and many of thelaser system modules and components for either of these three types oflasers. Applicants refer to this platform as their “three wavelengthplatform” since the three laser designs produce laser beams withwavelengths of about 248 nm for KrF, about 193 nm for ArF and about157.63 for F₂. This platform is also designed with interface componentsto make the laser systems at each of the three wavelengths compatiblewith modern lithography tools of all the major makers of such tools.Preferred F₂ product options include:

Rep Rate Pulse Energy Pulse Duration 4 kHz  7 mJ 24 ns 4 kHz 10 mJ 24 ns4 kHz 12 mJ 12 ns

The major components of this preferred laser system 2 are identified inFIG. 1. These include:

(1) laser system frame 4 which is designed to house all modules of thelaser except the AC/DC power supply module,

(2) the AC/DC high voltage power supply module 6,

(3) a resonant charger module 7 for charging two charging capacitorbanks to about 1000 volts at rates of 4000 charges per second,

(4) two commutator modules 8A and 8B each comprising one of the chargingcapacitor banks referred to above and each comprising a commutatorcircuit for forming very short high voltage electrical pulses, of about16,000 volts and about 1 μs duration from the energy stored on thecharging capacitor banks,

(5) two discharge chamber modules mounted in a top bottom configurationin frame 4 consisting of a master oscillator module 10 and a poweramplifier module 12. Each module includes a discharge chamber 10A and12A and a compression head 10B and 12B mounted on top of the chamber.The compression head compresses (time-wise) the electrical pulses fromthe commutator module from about 1 μs to about 50 ns with acorresponding increase in current,

(6) master oscillator optics including rear mirror 100 and outputcoupler 10C,

(7) beam control optics 14 including optics and instruments fordirecting (and possibly shaping) the seed beam into the power amplifier,and monitoring the MO output power,

(8) beam monitor 16 including wavelength, and energy monitors,

(9) shutter module 18,

(10) an auxiliary modules including gas control module 20, a coolingwater distribution module 22 and an air ventilation module 24,

(11) a customer interface module 26,

(12) a laser control module 28, and

(13) two high voltage supply modules 31A and 31B,

(14) a status lamp 30

For some applications the laser system would preferably include a pulsestretching unit (not shown) but as described in U.S. patent applicationSer. No. 10/141,216 to stretch the pulse duration beyond about 12 ns anda beam delivery unit also as described in U.S. patent application Ser.No. 10/141,216.

The Master Oscillator

The master oscillator 10 shown in FIGS. 1 and 1C is in many ways similarto prior art ArF lasers such as described in the '884 patent and in U.S.Pat. No. 6,128,323 and the ArF laser described in U.S. patentapplication Ser. No. 09/854,097 except the chamber and resonant cavityoptics are configured for F2 laser operation at the 157.63 nm spectralrange. Also, the output pulse energy is much less than the 5 mJ of thetypical prior art ArF lasers. However, major improvements over the '323laser are provided to permit operation at 4000 Hz and greater. Thispreferred master oscillator is optimized for spectral performance withline selection based on master oscillator F₂ control. The masteroscillator comprises discharge chamber 10 as shown in FIG. 1 in whichare located a pair of elongated electrodes 10 A2 and 10A3, each about 50cm long and spaced apart by about 12 mm, as shown in FIG. 2. Anode 10A4is mounted on flow shaping anode support bar 10A6. Four separate finnedwater cooled heat exchanger units 10A-8 are provided. A tangential fan10A10 is driven by two motors (not shown) for providing a laser gas flowat a velocity of about 80 m/s between the electrodes. The chamberincludes window units (not shown) with CaF₂ windows positioned at about47° with the laser beam. An electrostatic filter unit having an intakeat the center of the chamber, filters a small portion of the gas flow asindicated at 11 in FIG. 2 and the cleaned gas is directed into windowunits in the manner described in U.S. Pat. No. 5,359,620 (incorporatedherein by reference) to keep discharge debris away from the windows. Thegain region of the master oscillator is created by discharges betweenthe electrodes through the laser gas which in this embodiment iscomprised of about 0.1% F₂ and the rest neon, helium or a combination ofhelium and neon. The gas flow clears the debris of each discharge fromthe discharge region prior to the next pulse. The resonant cavity iscreated at the output side by an output coupler 10C. The output coupleris comprised of an uncoated CaF₂ reflecting optic mounted perpendicularto the beam direction so as to reflect about 5% of the laser light atabout 157 nm and to pass about 90% of the 157 nm light. The oppositeboundary of the resonant cavity is a maximum reflection mirror 100 asshown in FIG. 1.

In preferred embodiments the main charging capacitor banks for both themaster oscillator and the power amplifier are charged in parallel so asto reduce jitter problems. This is desirable because the time for pulsecompression in the pulse compression circuits of the two pulse powersystems is dependent on the level of the charge of the chargingcapacitors. Preferably pulse energy output from the power amplifier iscontrolled on a pulse-to-pulse basis by adjustment of the chargingvoltage. This limits somewhat the use of voltage to control beamparameters of the master oscillator. However, laser gas pressure and F₂concentration can be easily controlled separately in each chamber toachieve desirable beam parameters over a wide range pulse energy.Bandwidth decreases with decreasing F₂ concentration and laser gaspressure. For the master oscillator the time between discharge andlight-out is a function of F₂ concentration (1 ns/kPa). A very importantelement of the present invention is based on Applicants' discovery thatthe weak 157.52 mn F₂ line can be suppressed virtually out of existenceby proper control of F2 concentration and total gas pressure in themaster oscillator. This discovery test results and control techniquesare described in detail below.

Power Amplifier

The power amplifier in each of the three embodiments is comprised of alaser chamber which is very similar to the corresponding masteroscillator discharge chamber. Having the two separate chambers allowsthe pulse energy and integrated energy in a series of pulses (calleddose) to be controlled, to a large extent, separately from wavelengthand bandwidth. This permits better dose stability. All of the componentsof the chamber are the same and are interchangeable during themanufacturing process. However, in operation, the gas pressure ispreferably substantially lower in the MO as compared to the PA. Thecompression head 12B of the power amplifier as shown in FIG. 1 is alsosubstantially identical in this embodiment to the 10B compression headand the components of the compression head are also interchangeableduring manufacture. Also, the electrode spacing in the PA is preferablyabout 10 mm (as compared to about 16.5 mm for the MO). This closeidentity of the chambers and the electrical components of the pulsepower systems helps assure that the timing characteristics of the pulseforming circuits are the same or substantially the same so that jitterproblems are minimized.

The power amplifier is configured for a single pass through thedischarge region of the power amplifier discharge chamber in the FIG. 1.The charging voltages preferably are selected on a pulse-to-pulse basisto maintain desired pulse and dose energies. F₂ concentration and lasergas pressure can be adjusted in the power amplifier to provide a desiredoperating range of charging voltage. This desired range can be selectedto produce a desired value of dE/dV since the change in energy withvoltage is a function of F₂ concentration and laser gas pressure. Thetiming of injections is preferable based on charging voltage. Thefrequency of injections is preferably high to keep laser chamberconditions relatively constant and can be continuous or nearlycontinuous. Some users of these embodiments may prefer larger durations(such as 2 hours) between F₂ injections.

Pulse Power Circuit

In the preferred embodiment shown in FIG. 1, the basic pulse powercircuits are similar to pulse power circuits of prior art excimer laserlight sources for lithography. However, separate pulse power circuitsdownstream of the charging capacitors are provided for each dischargechamber. Preferably a single resonant charger charges two chargingcapacitor banks connected in parallel to assure that both chargingcapacitor banks are charged to precisely the same voltage. Importantimprovements are also provided to regulate the temperature of componentsof the pulse power circuits. In preferred embodiments the temperaturesof the magnetic cores of saturable inductors are monitored and thetemperature signals are utilized in a feedback circuit to adjust therelative timing of the discharge in the two chambers. FIGS. 5A and 5Bshow important elements of a preferred basic pulse power circuit whichis used for the MO. The same basic circuit is also used for the PA.

Resonant Charger

A preferred resonant charger system is shown in FIG. 5B. The principalcircuit elements are:

I1—A three-phase power supply 300 with a constant DC current output.

C-1—A source capacitor 302 that is an order of magnitude or more largerthan the existing C₀ capacitor 42.

Q1, Q2, and Q3—Switches to control current flow for charging andmaintaining a regulated voltage on C₀.

D1, D2, and D3—Provides current single direction flow.

R1, and R2—Provides voltage feedback to the control circuitry.

R3—Allows for rapid discharge of the voltage on C₀ in the event of asmall over charge.

L1—Resonant inductor between C-1 capacitor 302 and C₀ capacitor banks 42to limit current flow and setup charge transfer timing.

Control Board 304—Commands Q1, Q2, and Q3 open and closed based uponcircuit feedback parameters.

This circuit includes switch Q2 and diode D3, together known as aDe-Qing switch. This switch improves the regulation of the circuit byallowing the control unit to short out the inductor during the resonantcharging process. This “de-qing” prevents additional energy stored inthe current of the charging inductor, L1, from being transferred tocapacitor C₀.

Prior to the need for a laser pulse the voltage on C-1 is charged to600-800 volts and switches Q1-Q3 are open. Upon command from the laser,Q1 would close. At this time current would flow from C-1 to C₀ throughthe charge inductor L1. As described in the previous section, acalculator on the control board would evaluate the voltage on C₀ and thecurrent flowing in L1 relative to a command voltage set point from thelaser. Q1 will open when the voltage on the CO capacitor banks plus theequivalent energy stored in inductor L1 equals the desired commandvoltage. The calculation is:

V _(f) =[V _(C0s) ²+((L ₁ *I _(L1s) ²)/C ₀)]^(0.5)

Where:

V_(f)=The voltage on C₀ after Q1 opens and the current in L1 goes tozero.

V_(C0s)=The voltage on C₀ when Q1 opens.

I_(L1s)=The current flowing through L₁ when Q1 opens.

After Q1 opens the energy stored in L1 starts transferring to the COcapacitor banks through D2 until the voltage on the CO capacitor banksapproximately equals the command voltage. At this time Q2 closes andcurrent stops flowing to CO and is directed through D3. In addition tothe “de-qing” circuit, Q3 and R3 from a bleed-down circuit allowadditional fine regulation of the voltage on CO.

Switch Q3 of bleed down circuit 216 will be commanded closed by thecontrol board when current flowing through inductor L1 stops and thevoltage on C₀ will be bled down to the desired control voltage; thenswitch Q3 is opened. The time constant of capacitor C_(o) and resistorR3 should be sufficiently fast to bleed down capacitor C_(o) to thecommand voltage without being an appreciable amount of the total chargecycle.

As a result, the resonant charger can be configured with three levels ofregulation control. Somewhat crude regulation is provided by the energycalculator and the opening of switch Q1 during the charging cycle. Asthe voltage on the CO capacitor banks nears the target value, thede-qing switch is closed, stopping the resonant charging when thevoltage on C_(o) is at or slightly above the target value. In apreferred embodiment, the switch Q1 and the de-qing switch is used toprovide regulation with accuracy better than +/−0.1%. If additionalregulation is required, the third control over the voltage regulationcould be utilized. This is the bleed-down circuit of switch Q3 and R3(shown at 216 in FIG. 5B) to discharge the CO's down to the precisetarget value.

Improvements Downstream of the CO's

As indicated above, the pulse power system of the MO and the PA of thepresent invention each utilizes the same basic design (FIG. 5A) as wasused in the prior art systems. However, some significant improvements inthat basic design were required for the approximate factor of 3 increasein heat load resulting from the greatly increased repetition rate. Theseimprovements are discussed below.

Detailed Commutator and Compression Head Description

In this section, we describe details of fabrication of the commutatorand the compression head.

Solid State Switch

Solid state switch 46 is an P/N CM 800 HA-34H IGBT switch provided byPowerex, Inc. with offices in Youngwood, Pa. In a preferred embodiment,two such switches are used in parallel.

Inductors

Inductors 48, 54 and 64 are saturable inductors similiar to those usedin prior systems as described in U.S. Pat. Nos. 5,448,580 and 5,315,611.FIG. 6 shows a preferred design of the L_(o) inductor 48. In thisinductor four conductors from the two IGBT switches 46B pass throughsixteen ferrite toroids 49 to form part 48A an 8 inch long hollowcylinder of very high permability material with an ID of about 1 inchand an Od of about 1.5 inch. Each of the four conductors are thenwrapped twice around an insulating doughnut shaped core to form part48B. The four conductors then connect to a plate which is in turnconnected to the high voltage side of the C₁ capacitor bank 52.

A preferred sketch of saturable inductor 54 is shown in FIG. 8. In thiscase, the inductor is a single turn geometry where the assembly top andbottom lids 541 and 542 and center mandrel 543, all at high voltage,form the single turn through the inductor magnetic cores. The outerhousing 545 is at ground potential. The magnetic cores are 0.0005″ thicktape wound 50-50% Ni—Fe alloy provided by Magnetics of Butler, Pa. orNational Arnold of Adelanto, Calif. Fins 546 on the inductor housingfacilitate transfer of internally dissipated heat to forced air cooling.In addition, a ceramic disk (not shown) is mounted underneath thereactor bottom lid to help transfer heat from the center section of theassembly to the module chassis base plate. FIG. 8 also shows the highvoltage connections to one of the capacitors of the C₁ capacitor bank 52and to a high voltage lead on one of the induction units of the 1:25step up pulse transformer 56. The housing 545 is connected to the groundlead of unit 56.

A top and section view of the saturable inductor 64 is shownrespectively in FIGS. 9A and 9B. In the inductors of this embodiment,flux excluding metal pieces 301, 302, 303 and 304 are added as shown inFIG. 9B in order to reduce the leakage flux in the inductors. These fluxexcluding pieces substantially reduce the area which the magnetic fluxcan penetrate and therefore help to minimize the saturated inductance ofthe inductor. The current makes five loops through vertical conductorrods in the inductor assembly around magnetic core 307. The currententers at 305 travels down a large diameter conductor in the centerlabeled “1” and up six smaller conductors on the circumference alsolabeled “1” as shown in FIG. 9A. The current then flows down twoconductors labeled 2 on the inside, then up the six conductors labeled 2on the outside then down flux exclusion metal on the inside then up thesix conductors labeled 3 on the outside, then down the two conductorslabeled 3 on the inside, then up the six conductors labeled 4 on theoutside, then down the conductor labeled 4 on the inside. The fluxexclusion metal components are held at half the full pulsed voltageacross the conductor allowing a reduction in the safe hold-off spacingbetween the flux exclusion metal parts and the metal rods of the otherturns. The magnetic core 307 is made up of three coils 307A, B and Cformed by windings of 0.0005″ thick tape 80-20% Ni—Fe alloy provided byMagnetics, Inc. of Butler, Pa. or National Arnold of Adelanto, Calif.The reader should note that nano-crystoline materials such as VITROPERM™available from VACUUM SCHITELZE GmbH, Germany and FINEMET™ from HitachiMetals, Japan could be used for inductors 54 and 64. In prior art pulsepower systems, oil leakage from electrical components has been apotential problem. In this preferred embodiment, oil insulatedcomponents are limited to the saturable inductors. Furthermore, thesaturable inductor 64 as shown in FIG. 9B is housed in a pot type oilcontaining housing in which all seal connections are located above theoil level to substantially eliminate the possibility of oil leakage. Forexample, the lowest seal in inductor 64 is shown at 308 in FIG. 8B.Since the normal oil level is below the top lip of the housing 306, itis almost impossible for oil to leak outside the assembly as long as thehousing is maintained in an upright condition.

Capacitors

Capacitor banks 42, 52, 62 and 82 (i.e., C_(o), C₁, C_(p-1) and C_(p))as shown in FIG. 5 are all comprised of banks of off-the-shelfcapacitors connected in parallel. Capacitors 42 and 52 are film typecapacitors available from suppliers such as Vishay Roederstein withoffices in Statesville, N.C. or Wima of Germany. Applicants preferredmethod of connecting the capacitors and inductors is to solder them topositive and negative terminals on special printed circuit board havingheavy nickel coated copper leads in a manner similar to that describedin U.S. Pat. No. 5,448,580. Capacitor bank 62 and 64 is typicallycomposed of a parallel array of high voltage ceramic capacitors fromvendors such as Murata or TDK, both of Japan. In a preferred embodimentfor use on this ArF laser, capacitor bank 82 (i.e., C_(p)) comprised ofa bank of thirty three 0.3 nF capacitors for a capacitance of 9.9 nF;C_(p-1) is comprised of a bank of twenty four 0.40 nF capacitors for atotal capacitance of 9.6 nF; C₁ is a 5.7 μF capacitor bank and C_(o) isa 5.3 μF capacitor bank.

Pulse Transformer

Pulse transformer 56 is also similar to the pulse transformer describedin U.S. Pat. Nos. 5,448,580 and 5,313,481; however, the pulsetransformers of the present embodiment has only a single turn in thesecondary winding and 24 induction units equivalent to {fraction (1/24)}of a single primary turn for an equivalent step-up ratio of 1:24. Adrawing of pulse transformer 56 is shown in FIG. 10. Each of the 24induction units comprise an aluminum spool 56A having two flanges (eachwith a flat edge with threaded bolt holes) which are bolted to positiveand negative terminals on printed circuit board 56B as shown along thebottom edge of FIG. 10. (The negative terminals are the high voltageterminals of the twenty four primary windings.) Insulators 56C separatesthe positive terminal of each spool from the negative terminal of theadjacent spool. Between the flanges of the spool is a hollow cylinder1{fraction (1/16)} inches long with a 0.875 OD with a wall thickness ofabout {fraction (1/32)} inch. The spool is wrapped with one inch wide,0.7 mil thick Metglas™ 2605 S3A and a 0.1 mil thick mylar film until theOD of the insulated Metglas™ wrapping is 2.24 inches. A prospective viewof a single wrapped spool forming one primary winding is shown in FIG.10A.

The secondary of the transformer is a single OD stainless steel rodmounted within a tight fitting insulating tube of PTFE (Teflon®). Thewinding is in four sections as shown in FIG. 10. The low voltage end ofstainless steel secondary shown as 56D in FIG. 10 is tied to the primaryHV lead on printed circuit board 56B at 56E, the high voltage terminalis shown at 56F. As a result, the transformer assumes anauto-transformer configuration and the step-up ratio becomes 1:25instead of 1:24. Thus, an approximately −1400 volt pulse between the +and − terminals of the induction units will produce an approximately−35,000 volt pulse at terminal 56F on the secondary side. This singleturn secondary winding design provides very low leakage inductancepermitting extremely fast output rise time.

Details of Laser Chamber Electrical Components

The Cp capacitor 82 is comprised of a bank of thirty-three 0.3 nfcapacitors mounted on top of the chamber pressure vessel. (Typically anArF laser is operated with a lasing gas made up of 3.5% argon, 0.1%fluorine, and the remainder neon.) The electrodes are about 28 incheslong which are separated by about 0.5 to 1.0 inch preferably about ⅝inch. Preferred electrodes are described below. In this embodiment, thetop electrode is referred to as the cathode and the bottom electrode isconnected to ground as indicated in FIG. 5 and is referred to as theanode.

Discharge Timing

In ArF, KrF and F₂ electric discharge lasers, the electric dischargelasts only about 50 ns (i.e., 50 billionths of a second). This dischargecreates a population inversion necessary for lasing action but theinversion only exists during the time of the discharge. Therefore, animportant requirement for an injection seeded ArF, KrF or F₂ laser is toassure that the seed beam from the master oscillator passes throughdischarge region of the power amplifier during the approximately 50billionth of a second when the population is inverted in the laser gasso that amplification of the seed beam can occur. An important obstacleto precise timing of the discharge is the fact that there is a delay ofabout 5 microseconds between the time switch 42 (as shown in FIG. 5) istriggered to close and the beginning of the discharge which lasts onlyabout 40-50 ns. It takes this approximately 5 microseconds time intervalfor the pulse to ring through the circuit between the C₀'s and theelectrodes. This time interval varies substantially with the magnitudeof the charging voltage and with the temperature of the inductors in thecircuit.

Nevertheless in the preferred embodiment of the present inventiondescribed herein, Applicants have developed electrical pulse powercircuits that provide timing control of the discharges of the twodischarge chambers within a relative accuracy of less than about 2 ns(i.e., 2 billionths of a second). A block diagram of the two preferredcircuits are shown in FIG. 4.

Applicants have conducted tests which show that timing varies withcharging voltage by approximately 5-10 ns/volt. This places a stringentrequirement on the accuracy and repeatability of the high voltage powersupply charging the charging capacitors. For example, if timing controlof 5 ns is desired, with a shift sensitivity of 10 ns per volt, then theresolution accuracy would be 0.5 Volts. For a nominal charging voltageof 1000 V, this would require a charging accuracy of 0.05% which is verydifficult to achieve especially when the capacitors must be charged tothose specific values 4000 times per second.

Applicants' preferred solution to this problem is to charge the chargingcapacitor of both the MO and the PA in parallel from the single resonantcharger 7 as indicated in FIG. 1 and FIG. 4 and as described above. Itis also important to design the two pulse compression/amplificationcircuits for the two systems so that time delay versus charging voltagecurves match as shown in FIG. 4A. This is done most easily by using tothe extent possible the same components in each circuit.

Thus, in order to minimize timing variations (the variations arereferred to as jitter) in this preferred embodiment, Applicants havedesigned pulse power components for both discharge chambers with similarcomponents and have confirmed that the time delay versus voltage curvesdo in fact track each other as indicated in FIG. 4A. Applicants haveconfirmed that over the normal operating range of charging voltage,there is a substantial change in time delay with voltage but the changewith voltage is virtually the same for both circuits. Thus, with bothcharging capacitors charged in parallel charging voltages can be variedover a wide operating range without changing the relative timing of thedischarges.

Temperature control of electrical components in the pulse power circuitis also important since temperature variations can affect pulsecompression timing (especially temperature changes in the saturableinductors). Therefore, a design goal is to minimize temperaturevariations and a second approach is to monitor temperature of thetemperature sensitive components and using a feedback control adjust thetrigger timing to compensate. Controls can be provided with a processorprogrammed with a learning algorithm to make adjustments based onhistorical data relating to past timing variations with known operatinghistories. This historical data is then applied to anticipate timingchanges based on the current operation of the laser system.

Trigger Control

The triggering of the discharge for each of the two chambers isaccomplished separately utilizing for each circuit a trigger circuitsuch as one of those described in U.S. Pat. No. 6,016,325. Thesecircuits add timing delays to correct for variations in charging voltageand temperature changes in the electrical components of the pulse powerso that the time between trigger and discharge is held as constant asfeasible. As indicated above, since the two circuits are basically thesame, the variations after correction are almost equal (i.e., withinabout 2 ns of each other).

As indicated in FIGS. 6C, D, and E, performance of this preferredembodiment is greatly enhanced if the discharge in the power amplifieroccurs about 40 to 50 ns after the discharge in the master oscillator.This is because it takes several nanoseconds for the laser pulse todevelop in the master oscillator and another several nanoseconds for thefront part of the laser beam from the oscillator to reach the amplifierand because the rear end of the laser pulse from the master oscillatoris at a much narrower bandwidth than the front part. For this reason,separate trigger signals are provided to trigger switch 46 for eachchamber. The actual delay is chosen to achieve desired beam qualitybased on actual performance curves such as those shown in FIGS. 6C, Dand E. The reader should note, for example, that narrower bandwidth andlonger pulses can be obtained at the expense of pulse energy byincreasing the delay between MO trigger and PA trigger.

Other Techniques to Control Discharge Timing

Since the relative timing of the discharges can have important effectson beam quality as indicated in the FIGS. 6C, D and E graphs, additionalsteps may be justified to control the discharge timing. For example,some modes of laser operation may result in wide swings in chargingvoltage or wide swings in inductor temperature. These wide swings couldcomplicate discharge timing control.

Monitor Timing

The timing of the discharges can be monitored on a pulse-to-pulse basisand the time difference can be used in a feedback control system toadjust timing of the trigger signals closing switch 42. Preferably, thePA discharge would be monitored using a photocell to observe dischargefluorescence (called ASE) rather than the laser pulse since very poortiming could result if no laser beam being produced in the PA. For theMO either the ASE or the seed laser pulse could be used.

Bias Voltage Adjustment

The pulse timing can be increased or decreased by adjusting the biascurrents through inductors L_(B1) L_(B2) and L_(B3) which provide biasfor inductors 48, 54 and 64 as shown in FIG. 5. Other techniques couldbe used to increase the time needed to saturate these inductors. Forexample, the core material can be mechanically separated with a veryfast responding PZT element which can be feedback controlled based on afeedback signal from a pulse timing monitor.

Adjustable Parasitic Load

An adjustable parasitic load could be added to either or both of thepulse power circuits downstream of the CO's.

Additional Feedback Control

Charging voltage and inductor temperature signals, in addition to thepulse timing monitor signals can be used in feedback controls to adjustthe bias voltage or core mechanical separation as indicated above inaddition to the adjustment of the trigger timing as described above.

Burst Type Operation

Feedback control of the timing is relatively easy and effective when thelaser is operating on a continuous basis. However, normally lithographylasers operate in a burst mode such as the following to process 20 areason each of many wafers:

Off for 1 minute to move a wafer into place

4000 Hz for 0.2 seconds to illuminate area 1

Off for 0.3 seconds to move to area 2

4000 Hz for 0.2 seconds to illuminate area 2

Off for 0.3 seconds to move to area 3

4000 Hz for 0.2 seconds to illuminate area 3

. . .

4000 Hz for 0.2 seconds to illuminate area 199

Off for 0.3 seconds to move to area 200

4000 Hz for 0.2 seconds to illuminate area 200

Off for one minute to change wafers

4000 Hz for 0.2 seconds to illuminate area 1 on the next wafer, etc.

This process may be repeated for many hours, but will be interruptedfrom time-to-time for periods longer than 1 minute.

The length of down times will affect the relative timing between thepulse power systems of the MO and the PA and adjustment may be requiredin the trigger control to assure that the discharge in the PA occurswhen the seed beam from the MO is at the desired location. By monitoringthe discharges and the timing of light out from each chamber the laseroperator can adjust the trigger timing (accurate to within about 2 ns)to achieve best performance.

Preferably a laser control processor is programmed to monitor the timingand beam quality and adjust the timing automatically for bestperformance. Timing algorithms which develop sets of bin valuesapplicable to various sets of operating modes are utilized in preferredembodiments of this invention. These algorithms are in preferredembodiments designed to switch to a feedback control during continuousoperation where the timing values for the current pulse is set based onfeedback data collected for one or more preceding pulse (such as theimmediately preceding pulse).

Detailed Description of Timing Control

A more detailed description of preferred discharge timing circuit isprovided in U.S. patent application Ser. No. 10/210,716, which isincorporated by reference herein.

No Output Discharge

Timing algorithms such as those discussed above work very well forcontinuous or regularly repeated operation. However, the accuracy of thetiming may not be good in unusual situations such as the first pulseafter the laser is off for an unusual period of time such as 5 minutes.In some situations imprecise timing for the first one or two pulses of aburst may not pose a problem. A preferred technique is to preprogram thelaser so that the discharges of the MO and the PA are intentionally outof sequence for one or two pulses so that amplification of the seed beamfrom the MO is impossible. For example, laser could be programmed totrigger the discharge of the PA 80 ns prior to the trigger of the MO. Inthis case, there will be no significant output from the laser but thelaser metrology sensors can determine the timing parameters so that thetiming parameters for the first output pulse is precise. Alternatively,the MO could be triggered early enough relative to the triggering of thePA so that the MO beam passes through the PA prior to the PA discharge.FIGS. 4D, 4D1, 4E and 4E1 are flow charts showing possible controlalgorithms using these techniques.

Line Selection by Control of MO F₂

As stated in the background section prior art F₂ laser system designedfor integrated circuit lithography have required special opticalequipment for reducing or eliminating the weak 157.52 nm F₂ spectralline so that a high quality laser beam at about 157.63 nm could beproduced. Preferred optical equipment for performing this line selectionwere spectral dispersing elements such as prisms or gratings. Thesecomponents separate the spectral components directionally so that theweak line can be blocked at an aperature. Another prior art techniquefor line selection is to utilize one or more etalons for selecting thestrong line. These components are effective; however, they addcomplication to the laser system and can lead to performance problems,especially at high laser power levels and high pulse repetition rates.For example, the prisms could heat up at high repetition rates producingdistortion affecting beam direction. Changes in beam directionsespecially within the resonant cavity can have substantial adverseeffects of energy stability. Etalons can also be subject to thermaleffects creating serious beam quality problems.

Applicants' Discovery

Applicants have discovered that they can completely eliminate the weakF₂ line (at least as seen by typical laser beam spectrometers) withoutline selecting optics merely by careful control of the F2 concentrationand/or total gas pressure in the master oscillator. These tests haveconfirmed that the weak line intensity (to the extent it remains) isless than 0.01% (10⁻)⁴ of the strong line with the strong line havingsufficient intensity to adequately seed the power amplifier. Uponamplification in the PA he WL/SL ratio increases to +ON(10⁻³).Applicants tests have confirmed that MOPA systems can be built whichproduce narrowband F2 laser beams at 157.63 nm with pulse energiesgreater than 10 mJ per pulse at repetition rates of 4,000 Hz or greater.This represents a 40 Watt, line selected, narrow band (at less than 0.6pm), 157.63 nm F₂ laser beam.

FIG. 1A is an experimental set-up used by Applicants to confirm theirdiscoveries and prove the advantages of the present invention. Theset-up included master oscillator 10 and power amplifier 12 which weresubstantially equivalent to those components described above withreference to FIG. 1. The set-up also included a first test box 19 and asecond test box 2 each of which contained beam monitoring equipment 23Aand 23B for measuring pulse energy each of chambers, beam pointing andbeam profile. The second test box contained a spectrometer which couldmonitor spectrum of the output beam 25 and could also monitor thespectrum of the MO by interchanging the output coupler 10C and the highreflection mirror 100. The results of these tests are shown in FIGS.24A-F.

MO F₂ Pressure and Chamber Pressure vs. MO Output

FIG. 24A shows master oscillator pulse energy and MO FWHM bandwidth vs.MO total chamber pressure in kilo Pascals (kPa) with 0.15 kPa of F₂ inthe chamber and with 0.3 kPa of F₂ in the chamber. This figure alsoshows MO output pulse energy vs. F₂ pressure and total pressure (withhelium buffer). MO pulse energies greater than 0.5 mJ can be achievedwith F₂ partial pressure as low as about 0.15 kPa and a total gaspressure (with H_(e) buffer) as low as 200 kPa. This graph (i.e., FIG.24A) also shows that in these low F₂ concentrations and total gaspressures the F₂ bandwidth is very narrow, i.e. about 0.5 pm.

FIG. 24B shows the ratio of the intensity of the weak line to theintensity of the strong line as a function of chamber total pressurewith MO F₂ partial pressure of 0.15 kPa, 0.20 kPa, 0.25 kPa an 0.30 kPa.(The reader should note that the F₂ kPa values shown on the graphsrepresent partial pressure of F₂—He mixtures which is 1.0 percent F₂ and99 percent He).

MO Input vs. PA Output

FIG. 24C shows that from 0.15 kPa to 0.70 kPa in the power amplifierchamber and a total chamber pressure (with Helium buffer) of 500 kPaonly about 0.03 mJ to 0.05 mJ of MO output pulse energy is needed toproduce saturation in the power amplifier.

Other Test Data

FIG. 24D shows the FWHM bandwidth as a function of MO total pressure forthree specific F₂ partial pressures and FIG. 24E shows the weak line tostrong line ratio as a function of total MO chamber pressure for fourcombinations of MO and PA F₂ partial pressures. FIG. 24F shows MOPApulse energy, weak line strong line ratios as a function of dischargetiming deviation from “best timing” (i.e., the discharge timing thatproduces maximum pulse energy output). FIG. 24G is a graph which showspulse energy output and bandwidth as a function of discharge timing.FIG. 24H shows energy output and the weak line strong line ratio. FIG.24I shows energy out and system pointing as a function of masteroscillator pointing.

FIG. 24J shows horizontal divergence for the master oscillator and thesystem as a function of pointing angle and FIG. 24K shows the same thingfor the vertical direction. FIG. 24L shows the spectrum for the systemoutput and the ASE of the power amplifier. (Note the weak 157.52 F₂ lineis clearly evident in the ASE but is not present in a detectableintensity in the output laser beam.)

Water Cooling of Components

To accommodate greater heat loads water cooling of pulse powercomponents is provided in addition to the normal forced air coolingprovided by cooling fans inside the laser cabinet in order to supportoperation pulse rates of 4 KHz or greater.

One disadvantage of water cooling has traditionally been the possibilityof leaks near the electrical components or high voltage wiring. Thisspecific embodiment substantially avoids that potential issue byutilizing a single solid piece of cooling tubing that is routed within amodule to cool those components that normally dissipate the majority ofthe heat deposited in the module. Since no joints or connections existinside the module enclosure and the cooling tubing is a continuous pieceof solid metal (e.g. copper, stainless steel, etc.), the chances of aleak occurring within the module are greatly diminished. Moduleconnections to the cooling water are therefore made outside the assemblysheet metal enclosure where the cooling tubing mates with aquick-disconnect type connector.

Saturable Inductor

In the case of the commutator module a water cooled saturable inductor54A is provided as shown in FIG. 11 which is similar to the inductor 54shown in FIG. 8 except the fins of 54 are replaced with a water cooledjacket 54A1 as shown in FIG. 11. The cooling line 54A2 is routed withinthe module to wrap around jacket 54A1 and through aluminum base platewhere the IGBT switches and Series diodes are mounted. These threecomponents make up the majority of the power dissipation within themodule. Other items that also dissipate heat (snubber diodes andresistors, capacitors, etc.) are cooled by forced air provided by thetwo fans in the rear of the module.

Since the jacket 54A1 is held at ground potential, there are no voltageisolation issues in directly attaching the cooling tubing to the reactorhousing. This is done by press-fitting the tubing into a dovetail groovecut in the outside of the housing as shown at 54A3 and using a thermallyconductive compound to aid in making good thermal contact between thecooling tubing and the housing.

Cooling High Voltage Components

Although the IGBT switches “float” at high voltage, they are mounted onan aluminum base electrically isolated from the switches by a {fraction(1/16)} inch thick alumina plate. The aluminum base plate whichfunctions as a heat sink and operates at ground potential and is mucheasier to cool since high voltage isolation is not required in thecooling circuit. A drawing of a water cooled aluminum base plate isshown in FIG. 7. In this case, the cooling tubing is pressed into agroove in an aluminum base on which the IGBT's are mounted. As with theinductor 54 a, thermally conductive compound is used to improve theoverall joint between the tubing and the base plate.

The series diodes also “float” at high potential during normaloperation. In this case, the diode housing typically used in the designprovides no high voltage isolation. To provide this necessaryinsulation, the diode “hockey puck” package is clamped within a heatsink assembly which is then mounted on top of a ceramic base that isthen mounted on top of the water-cooled aluminum base plate. The ceramicbase is just thick enough to provide the necessary electrical isolationbut not too thick to incur more than necessary thermal impedance. Forthis specific design, the ceramic is {fraction (1/16)}″ thick aluminaalthough other more exotic materials, such as beryllia, can also be usedto further reduce the thermal impedance between the diode junction andthe cooling water.

A second embodiment of a water cooled commutator utilizes a single coldplate assembly which is attached to the chassis baseplate for the IGBT'sand the diodes. The cold plate may be fabricated by brazing single piecenickel tubing to two aluminum “top” and “bottom” plates. As describedabove, the IGBT's and diodes are designed to transfer their heat intothe cold plate by use of the previously mentioned ceramic disksunderneath the assembly. In a preferred embodiment of this invention,the cold plate cooling method is also used to cool the IGBT and thediodes in the resonant charger. Thermally conductive rods or a heat pipecan also be used to transfer heat from the outside housing to thechassis plate.

Detailed Compression Head Description

The water-cooled compression head is similar in the electrical design toa prior art air-cooled version (the same type ceramic capacitors areused and similar material is used in the reactor designs). The primarydifferences in this case are that the module must run at higherrep-rates and therefore, higher average power. In the case of thecompression head module, the majority of the heat is dissipated withinthe modified saturable inductor 64A. Cooling the subassembly is not asimple matter since the entire housing operates with short pulses ofvery high voltages. The solution to this issue as shown in FIGS. 12, 12Aand 12B is to inductively isolate the housing from ground potential.This inductance is provided by wrapping the cooling tubing around twocylindrical forms that contain a ferrite magnetic core. Both the inputand output cooling lines are coiled around cylindrical portions of aferrite core formed of the two cylindrical portions and the two ferriteblocks as shown in FIGS. 12, 12A and 12B.

The ferrite pieces are made from CN-20 material manufactured by CeramicMagnetics, Inc. of Fairfield, N.J. A single piece of copper tubing(0.187″ diameter) is press fit and wound onto one winding form, aroundthe housing 64A1 of inductor 64A and around the second winding form.Sufficient length is left at the ends to extend through fittings in thecompression head sheet metal cover such that no cooling tubing jointsexist within the chassis.

The inductor 64A comprises a dovetail groove as shown at 64A2 similar tothat used in the water-cooled commutator first stage reactor housing.This housing is much the same as previous air-cooled versions with theexception of the dovetail groove. The copper cooling-water tubing ispress fit into this groove in order to make a good thermal connectionbetween the housing and the cooling-water tubing. Thermally conductivecompound is also added to minimize the thermal impedance.

The electrical design of inductor 64A is changed slightly from that of64 shown in FIGS. 9A and 9B. Inductor 64A provides only two loops(instead of five loops) around magnetic core 64A3 which is comprised offour coils of tape (instead of three).

As a result of this water-cooled tubing conductive path from the outputpotential to ground, the bias current circuit is now slightly different.As before, bias current is supplied by a dc—dc converter in thecommutator through a cable into the compression head. The current passesthrough the “positive” bias inductor L_(B2) and is connected to the Cp-1voltage node. The current then splits with a portion returning to thecommutator through the HV cable (passing through the transformersecondary to ground and back to the dc—dc converter). The other portionpasses through the compression head reactor Lp-1 (to bias the magneticswitch) and then through the cooling-water tubing “negative” biasinductor L_(B3) and back to ground and the dc—dc converter. By balancingthe resistance in each leg, the designer is able to ensure thatsufficient bias current is available for both the compression headreactor and the commutator transformer.

The “positive” bias inductor L_(B2) is made very similarly to the“negative” bias inductor L_(B3). In this case, the same ferrite bars andblocks are used as a magnetic core. However, two 0.125″ thick plasticspacers are used to create an air gap in the magnetic circuit so thatthe cores do not saturate with the dc current. Instead of winding theinductor with cooling-water tubing, 18 AWG teflon wire is wound aroundthe forms.

Quick Connections

In this preferred embodiment, three of the pulse power electricalmodules utilize blind mate electrical connections so that all electricalconnections to the portions of the laser system are made merely bysliding the module into its place in the laser cabinet. These are the ACdistribution module, the power supply module and the resonant chargesmodule. In each case a male or female plug on the module mates with theopposite sex plug mounted at the back of the cabinet. In each case twoapproximately 3-inch end tapered pins on the module guide the moduleinto its precise position so that the electrical plugs properly mate.The blind mate connectors such as AMP Model No. 194242-1 arecommercially available from AMP, Inc. with offices in Harrisburg, Pa. Inthis embodiment connectors are for the various power circuits such as208 volt AC, 400 volt AC, 1000 Volt DC (power supply out and resonantcharges in) and several signal voltages. These blind mate connectionspermit these modules to be removed for servicing and replacing in a fewseconds or minutes. In this embodiment blind mate connections are notused for the commutator module the output voltage of the module is inthe range of 20 to 30,000 volts. Instead, a typical high voltageconnector is used.

Discharge Components

FIGS. 13 and 13A(1) show details of an improved discharge configurationutilized in preferred embodiments of the present invention. Thisconfiguration includes an electrode configuration that Applicants call ablade-dielectric electrode. In this design, the anode 540 comprises ablunt blade shaped electrode 542 with dielectric spacers 544 mounted onboth sides of the anode as shown to improve the gas flow in thedischarge region. The spacers are attached to anode support bar 546 withscrews at each end of the spacers beyond the discharge region. Thescrews allow for thermal expansion slippage between the spacers and thebar. The anode is 26.4 inches long and 0.439 inches high. It is 0.284inches wide at the bottom and 0.141 inches wide at the top. It isattached to flow shaping anode support bar 546 with screws throughsockets that allow differential thermal expansion of the electrode fromits center position. The anode is comprised of a copper based alloypreferably C36000, C95400, or C19400. Cathode 541 has a cross sectionshape as shown in FIG. 13A. A preferred cathode material is C36000.Additional details of this blade dielectric configuration are providedin U.S. patent application Ser. No. 09/768,753 incorporated herein byreference. The current return 548 in this configuration is comprised ofa whale bone shaped part with 27 ribs equally spaced along the length ofelectrode 542, the cross section of which is shown in FIG. 13A (1). Asdescribed above, the current return is fabricated from sheet metal andthe whale-bone ribs (each having cross-section dimensions of about 0.15inch×0.09 inch) are twisted so that the long dimension of each rib is inthe direction of current flow.

An alternative dielectric spacer design for the anode is shown in FIG.13A2 to improve flow even more. In this case the spacers mate moreperfectly with the flow shaping anode support bar to provide a bettergas flow path. Applicants call this their “fast back” blade dielectricanode design.

Alternate Pulse Power Circuit

A second preferred pulse power circuit is shown in FIGS. 5C1, 5C2 and5C3. This circuit is similar to the one described above but utilizes ahigher voltage power supply for charging C₀ to a higher value. As in theabove described embodiments, a high voltage pulse power supply unitoperating from factory power at 230 or 460 volts AC, is power source fora fast charging resonant charger as described above and designed forprecise charging two 2.17 μF at frequencies of 4000 to 6000 Hz tovoltages in the range of about 1100 V to 2250 V. The electricalcomponents in the commutator and compression head for the masteroscillator are as identical as feasible to the corresponding componentsin the power amplifier. This is done to keep time responses in the twocircuits as identical as feasible. Switches 46 are banks of two IGBTswitches each rated at 3300 V and arranged in parallel. The C₀ capacitorbanks 42 is comprised of 128 0.068 μF 1600 V capacitors arranged in 64parallel legs to provide the 2.17 μF C₀ bank. The C₁ capacitor banks 52are comprised of 136 0.068 μF 1600 V capacitors arranged in 68 parallellegs to provide a bank capacitance of 2.33 μF. The C_(p-1) and C_(p)capacitor banks are the same as those described above with reference toFIG. 5. The 54 saturable inductors are single turn inductors providingsaturated inductance of about 3.3 nH with five cores comprised of 0.5inch thick 50%-50% Ni—Fe with 4.9 inch OD and 3.8 inch ID. The 64saturable inductors are two turn inductors providing saturatedinductance of about 38 nH each comprised of 5 cores, 0.5 inch thick madewith 80%-20% Ni—Fe with an OD of 5 inches and an ID of 2.28 inches.Trigger circuits are provided for closing IGBT's 46 with a timingaccuracy of two nanoseconds. The master oscillator is typicallytriggered about 40 ns prior to the triggering of the IGBT 46 for poweramplifier. However, the precise timing is preferably determined byfeedback signals from sensors which measure the timing of the output ofthe master oscillator and the power amplifier discharge.

Pulse Length

The output pulse length measured in tests conducted by Applicants onthese F₂ lasers is in the range of about 12 ns and is to some extent afunction of the relative timing of the two discharges. A longer pulselength (other things being equal) can increase the lifetime of opticalcomponents of lithography equipment.

Applicants have identified several techniques for increasing pulselength. As indicated above, the relative time between discharges can beoptimized for pulse length. The pulse power circuits of both the MO andthe PA could be optimized for longer pulses using techniques such asthose described in U.S. patent application Ser. No. 09/451,995incorporated herein by reference. An optical pulse multiplier systemsuch as one of those described in U.S. Pat. No. 6,067,311, incorporatedby reference herein, could be added downstream of the PA to reduce theintensity of individual pulses. A preferred pulse multiplier unit (alsocalled a pulse stretcher) is described in the next section. This pulsemultiplier could be made a part of the beam path to lens components of alithography tool. The chamber could be made longer and the electrodescould be configured to produce traveling wave discharges designed forlonger pulse lengths.

Pulse Multiplier Unit

A preferred pulse multiplier unit is shown in FIG. 22A. Light beam 20from laser 50 hits the beam splitter 22. Beam splitter has areflectivity of about 40%. About 40% of the light reflects a firstportion of the output beam 30. The rest of the incoming beam transmitsthrough the beam splitter 22 as beam 24. The beam is reflected back at asmall angle by a mirror 26, which is a spherical mirror with the focallength equal the distance from beam splitter 22 to the mirror. So, thebeam is focused to a point 27 near the beam splitter 22 but missing itslightly. This beam spreads again and is now reflected by mirror 28,which is also a spherical mirror with the focal length equal thedistance from this mirror to point 27. The mirror 28 reflect the beamback at a small angle and also collimates the reflected beam. Thisreflected beam 32 propagates to the right and is reflected by mirror 29to beam splitter 22 where about 60% of the beam is transmitted throughbeam splitter 22 to merge into and become the second portion of outputbeam 30. A portion (about 40%) of beam 34 is reflected by the beamsplitter 22 in the direction of beam 24 for a repeat of the trip of beam32. As a result, a short input pulse is split into several portions, sothat total duration of the beam is increased and its peak intensity isdecreased. Mirrors 26 and 28 create a relay system which images theportions of the outcoming beam onto each other. Because of that imaging,each portion of the output beam is virtually the same. (If mirrors 26and 28 were flat, beam divergence would spread the beam for eachsubsequent repetition, so beam size would be different for eachrepetition.) The total optical path length from beam splitter 22 tomirror 26 to mirror 28 to mirror 27 and, finally, to beam splitter 22determines the time delay between repetitions. FIG. 22B1 shows the pulseprofile of a typical pulse produced by an ArF excimer laser. (Theseresults can be applied to an F₂ laser except the un-multiplied pulse forthe F₂ laser is about 12 ns instead of about 18 ns for the ArF laser.)FIG. 22B2 shows the simulated output pulse profile of a similar ArFlaser pulse after being spread in a pulse stretcher built in accordancewith FIG. 6. In this example the T_(is) of the pulse was increased from18.16 ns to 45.78 ns. (T_(is) is a measure of pulse duration used fordescribing laser pulses. It refers to the integral square pulseduration.)

FIG. 22C shows a layout similar to the FIG. 22A layout but with anadditional delay path. In this case, the first beam splitter 22A isdesigned for a reflection of 25 percent and the second beam splitter 22Bis designed for a reflection of 40 percent. The resulting beam shapeproduced by computer simulation is shown in FIG. 22D. The T_(is) forthis stretched pulse is about 73.2 ns. In the FIG. 22C embodiment, theportions of the beam is transmitted through beam splitter 22B areflipped in orientation when they return and are joined into exit beam30. This reduces significantly the spatial coherence of the beam.

FIGS. 22E and F show beam splitter designs which use optical elementswithout coatings. FIG. 22E shows a beam splitter design to takeadvantage of frustrated internal reflection and FIG. 22F shows atransparent uncoated plate tilted to produce a Fresnel reflection fromboth sides of the plate to achieve the desired reflection-transmissionratio.

The pulse stretcher unit could be installed in the back of verticaloptical table 11 as suggested above or it could be installed on top ofthe table or even inside of it.

Pulse and Dose Energy Control

Pulse energy and dose energy are preferably controlled with a feedbackcontrol system and algorithm such as that described above. The pulseenergy monitor can be at the laser as closer to the wafer in thelithography tool. Using this technique charging voltages are chosen toproduce the pulse energy desired. In the above preferred embodiment,both the MO and the PA are provided with the same charging voltage sincethe CO's are charged in parallel.

As discussed above, Applicants have determined that this technique worksvery well and greatly minimize timing jitter problems. This technique,however, does reduce to an extent the laser operator's ability tocontrol the MO independently of the PA. However, there are a number ofoperating parameters of the MO and the PA that can be controlledseparably to optimize performance of each unit. These other parametersinclude: laser gas pressure, F₂ concentration and laser gas temperature,These parameters preferably are controlled independently in each of thetwo chambers and regulated in a processor controlled feedbackarrangement.

Optional Optical Quality Improvement

With this system the master oscillator to a large extent determines thewavelength and the bandwidth and the power amplifier primarily controlsthe pulse energy. The pulse energy needed for an efficient seeding ofthe power amplifier can be as low as a small fraction of a mJ as shownin FIG. 6B.

Total Internal Reflecting Spatial Filter

Spatial filtering is effective at reducing the integrated 95% bandwidth.However, all direct spatial filtering techniques previously proposedrequired at least concentrating the beam and in most cases actuallyfocusing the beam. Additionally all previous designs required multipleoptical elements. A simple, compact spatial filter, that does notrequire a focused beam, would be more readily adaptable forincorporation inside the laser resonator if spatial filtering isdesired.

A preferred filter is a single prism approximately 2 inches in length.The entrance and exit faces of the prism are parallel to each other andnormal to the incident beam. Two other faces would be parallel to eachother but orientated at an angle equal to the critical angle withrespect to the entrance and exit faces. At a wavelength of about 157 nmthe critical angle in CaF₂ is 39.89 degrees. The only coatings requiredwould be normal incidence anti-reflection coatings on the entrance andexit faces of the prism.

The spatial filter would work in the following manner. The beam wouldenter at normal incidence to the entrance face of the prism. The beamwould then propagate to the critical angle face of the prism. If thebeam was collimated all rays would be incident at the critical angle atthis second face. However, if the beam if diverging or converging someof the rays will strike this face at angles greater than and less thanthe critical angle. All rays striking this face at or greater than thecritical angle will be reflected at 100%. Rays striking this face at anangle less than the critical angle will be reflected at values less than100% and will be attenuated. All rays that are reflected will beincident at the opposite face of the prism at the same angle where theywill also be attenuated by the same amount. In the design proposed therewill be a total of six reflections for each pass. The reflectivity forp-polarized light at an angle of 1 mrad less than the critical angle isabout 71%. Therefore, all rays with incident angles that differ from thecritical angle by 1 mrad or more will be transmitted at the exit face atless than 13% of their original intensity.

However, a single pass of this filter will only be one sided. All raysthat are incident at angles greater than the critical angle reflect at100%. Once exiting the spatial filter prism, the beam will be incidentupon a mirror. Inside the laser resonator this mirror could be theoutput coupler. After reflecting off the mirror, the rays will re-enterthe spatial filter prism, but with one critical difference. All raysthat exited the spatial filter at angles that were greater than thecritical angle will be inverted after reflecting off the mirror. Theserays will now re-enter the prism at values less than the critical angleand will be attenuated. It is this second pass through the prism thatchanges the transmission function of the prism from a one sided filterinto a true bandpass filter.

FIG. 23B shows the design of the spatial filter. The input and outputfaces of the prism are ½ inch. The critical angle faces are about 2inches. The input beam width is 2.6 mm and represents the width of thebeam in the short axis. The prism would have a height of 1 inch in theplane of the drawing. The figure shows three sets of rays. The first setof rays is collimated and strikes the surfaces at the critical angle.These are the green rays. A second set of rays is incident at thesurface less than the critical angle and is terminated at the firstreflection. They are the blue rays. These rays are more visible in themagnified section. They represent the rays that are attenuated on thefirst pass. The final set of rays is incident at an angle greater thanthe critical angle. These rays propagate through the entire first passbut are terminated at the first reflection of the second pass. Theyrepresent the rays that are attenuated on the second pass.

Telescope Between Chambers

In preferred embodiments a cylindrical refractive telescope is providedbetween the output of the master oscillator and the input of the poweramplifier. This controls the horizontal size of the beam entering thepower amplifier. This telescope can also be designed using well knowntechniques to control the horizontal divergence.

Metrology

In preferred embodiments of the present invention pulse energy ismonitored controlled on a pulse to pulse basis with feedback from a fastphotodiode energy monitor. In many applications, pulse-by-pulsemonitoring of wavelength and bandwidth are not provided since thenatural centerline wavelength and bandwidth of the major F2 line isrelatively invariable. If desired, however, both wavelength andbandwidth could be monitored generally in the same manner as in priorart excimer lasers but at the 157 nm wavelength range.

Preferably power monitors (p-cells) should be provided at the output ofthe master oscillator, after the power amplifies and after the pulsemultiplies. Preferably a p-cell should also be provided for monitoringany back reflections into the master oscillator. Such back reflectionscould be amplified in the oscillator and damage the master oscillatoroptical components. The back reflection signal from the back reflectionmonitor is used to shut the laser down if a danger threshold isexceeded. Also, the system should be designed to avoid glint in the beampath that might cause any significant back reflection.

The beam parameter measurement and control for this laser is describedbelow. The wavemeter used in the present embodiment is similar to theone described in U.S. Pat. No. 5,978,394 and some of the descriptionbelow is extracted from that patent. At wavelengths in the range of 157nm, wavelength and bandwidth metrology components are subject toradiation damage so Applicants recommend that these measurements be madeperiodically rather than on a pulse to pulse basis. For example,wavelength and bandwidth could be monitored for 30 pulses once each 10minute period. At this rate the metrology components for the F2 lasersshould have lifetimes at least comparable to KrF and ArF lasers. Toaccomplish this a shutter should be provided for the wavemeter to blockthe beam access to the wavelength and bandwidth metrology components.

The optical equipment in these units measure pulse energy, wavelengthand bandwidth. These measurements are used with feedback circuits tomaintain pulse energy and wavelength within desired limits.

A small portion of the laser beam is reflected to an energy detectorwhich comprises a very fast photo diode which is able to measure theenergy of individual pulses occurring at the rate of 4,000 pulses persecond. The pulse energy is about 10 mJ, and the output of detector 69is fed to a computer controller which uses a special algorithm to adjustthe laser charging voltage to precisely control the pulse energy offuture pulses based on stored pulse energy data in order to limit thevariation of the energy of individual pulses and the integrated energyof bursts of pulses.

Based on the measurement of pulse energy of each pulse as describedabove, the pulse energy of subsequent pulses are controlled to maintaindesired pulse energies and also desired total integrated dose of aspecified number of pulses all as described in U.S. Pat. No. 6,005,879,Pulse Energy Control for Excimer Laser which is incorporated byreference herein.

Prism Output Coupler

The output coupler of gas discharge lasers configured as oscillators istypically a partially reflecting mirror which is usually a wedge shapedoptical element with one surface oriented transverse to the beam pathand coated to reflect a desired portion of the beam and transmit theremaining portion. The other surface is often coated with ananti-reflection coating and may be oriented as an angle other thantransverse to the beam path so that any reflections from this surface isnot returned to the gain region.

Coated surfaces sometimes provide lifetime problems when used in thesehigh intensity UV applications. A solution is shown in FIG. 17. In thiscase, the output coupler 120 is prism shaped. The front surface (closestto the gain region) is oriented for the lowest loss angle (forp-polarization) while the second surface is orthogonal to the refractedlaser beam to provide the reflected beam for amplification. This designeliminates the need for an AR coating and also provides some additionalspectral separation due to dispersions. For this F₂ application, theprism is comprised of CaF₂ with an apex angle of 32.7 degrees and anangle of incidence of 57.2 degrees. In this preferred embodiment, thereis no coating on the second surface and the approximately 4.7 percentFresnel reflection provides sufficient reflection for the masteroscillator.

Beam Steering with Optical Element

In spite of efforts to maintain constant conditions in the beam path,many laser operations such as burst mode operation, such as thosedescribed above, produce transient conditions which in some cases causesignificant transient steering of the output laser beam. This transientsteering can be corrected with an active beam direction control systemwhich includes a beam direction monitor and beam direction controlmechanism. In a preferred embodiment the beam direction monitor is asplit detector, also known as a bi-cell detector or segmented detector.This type of detector has two distinct photosensitive elements separatedby a small gap. The ratio of the outputs of the two elements is ameasure of the beam direction. The beam direction control mechanism canbe a pivot mirror preferably in the line selection package 10C in FIG.1. Alternatively, one of the prisms in the line selector unit shown inFIG. 16A could be pivoted. If all of the prisms in the selector unit aremounted on a prism plate, the plate itself could be pivoted. The driverproducing the pivot preferably is a piezoelectric driver or it could bea voice coil or a stepper motor drive unit or any of other similar driveunits. Controls for the direction control mechanism should preferablyinclude a processor programmed with an appropriate feedback algorithmand also additional electronic control and software permitting operatoradjustment of the beam direction.

Beam Steering Compensation with Purge Pressure

As indicated above, a small amount of beam steering in the beam path canresult from operations such as burst mode operation commonly used inlaser integrated circuit lithography. Even very minor changes in thebeam direction can be extremely undesirable. As indicated above,techniques can be utilized to eliminate the causes of beam steering.Also, pivoting of optical elements such as prisms or mirrors couldcorrect for unwanted changes in beam directions. Another approach is tocorrect for unwanted beam direction changes by controlling the purge gaspressure in portions of the beam path. In a preferred embodiment thepurge pressure in the line selection package is controlled to compensatefor beam direction changes. Applicants have determined that for thefive-prism line selector shown in FIG. 16A. The output beam directionfor this five-prism configuration is related to the purge gas pressurein the range of about 1 atm by the following factor: Δφ=15 milliradiansper atmosphere. Preferably the beam direction is monitored by a splitdetector as discussed above and a feedback signal regulates the pressurein the LSP by controlling a purge gas flow valve. Another approach is touse a temperature sensor to provide a feedback signal.

Gas Control

The preferred embodiment of this invention has a gas control module asindicated in FIG. 1 and it is configured to fill each chamber withappropriate quantities of laser gas. Preferably appropriate controls andprocessor equipment is provided to maintain continuous flow of gas intoeach chamber so as to maintain laser gas concentrations constant orapproximately constant at desired levels. This may be accomplished usingtechniques such as those described in U.S. Pat. No. 6,028,880 or U.S.Pat. No. 6,151,349 or U.S. Pat. No. 6,240,117 (both of which areincorporated hereby reference).

Another technique for providing continuous flow of laser gas into thechambers which Applicants call its binary fill technique is to provide anumber (such as 5) fill lines each successive line orificed to permitdouble the flow of the previous line with each line having a shut offvalve. The lowest flow line is orificed to permit minimum equilibriumgas flow. Almost any desired flow rate can be achieved by selectingappropriate combinations of valves to be opened. Preferably a buffertank is provided between the orificed lines and the laser gas sourcewhich is maintained at a pressure at about twice the pressure of thelaser chambers.

Vertical Optical Table

In preferred embodiments the two chambers and the laser optics aremounted on a vertically oriented optical table. The table is preferablysupported in the laser frame with a three-point kinematic mount. Onepreferred embodiment arrangement is shown in FIG. 1C1. Metal straps areprovided on table 11 at locations A, B, and C where the table is mountedto the laser frame 4 (not shown in FIG. 1C1). A swivel joint is providedat location A which anchors the table but permits it to swivel. A balland V-groove is provided at location B which restricts rotation in theplane of the bottom surface of the table and rotation in the plane ofthe table front surface. A ball and slot groove is provided at locationC which restricts rotation around the A-B axis.

Laser Chambers Four Kilo Hertz Operation

Preferred embodiments are designed to operate at pulse repetition ratesof 4,000 pulses per second. Clearing the discharge region of dischargeaffected gas between pulses requires a gas flow between the electrodes18A and 20A of up to about 67 m/s. To achieve these speeds, the diameterof tangential fan unit has been set at 5 inches (the length of the bladestructure is 26 inches) and the rotational speed has been increased. Toachieve this performance the embodiment utilizes two motors whichtogether deliver up to about 4 kw of drive power to the fan bladestructure. At a pulse rate of 4000 Hz, the discharge will add about 12kw of heat energy to the laser gas. To remove the heat produced by thedischarge along with the heat added by the fan four separate watercooled finned heat exchanger units 58A are provided. The motors and theheat exchangers are described in detail below.

A preferred embodiment of the present invention utilizes four finnedwater cooled heat exchangers 58A shown generally in FIG. 4. Each ofthese heat exchangers is somewhat similar to the single heat exchangersshown at 58 in FIG. 1 having however substantial improvements.

Heat Exchanger Components

A cross sectional drawing of one of the heat exchangers is shown in FIG.21. The middle section of the heat exchanger is cut out but both endsare shown. FIG. 21A shows an enlarged view of the end of the heatexchanger which accommodates thermal expansion and contraction.

The components of the heat exchanger includes a finned structure 302which is machined from solid copper (CU 11000) and contains twelve fins303 per inch. Water flow is through an axial passage having a borediameter of 0.33 inch. A plastic turbulator 306 located in the axialpassage prevents stratification of water in the passage and prevents theformation of a hot boundary layer on the inside surface of the passage.A flexible flange unit 304 is a welded unit comprised of inner flange304A, bellows 304B and outer flange 304C. The heat exchanger unitincludes three c-seals 308 to seal the water flowing in the heatexchanger from the laser gas. Bellows 304B permits expansion andcontraction of the heat exchanger relative to the chamber. A double portnut 400 connects the heat exchanger passage to a standard {fraction(5/16)} inch positional elbow pipe fitting which in turn is connected toa water source. O-ring 402 provides a seal between nut 400 and finnedstructure 302. In preferred embodiments cooling flow direction in two ofthe units is opposite the other two minimizing axial temperaturegradients.

The Turbulator

In a preferred embodiment, the turbulator is comprised of fouroff-the-shelf, long in-line mixing elements which are typically used tomix epoxy components and are available from 3M Corporation (StaticMixer, Part No. 06-D1229-00). The in-line mixers are shown at 306 inFIGS. 21 and 21A. The in-line mixers force the water to flow along agenerally helical path which reverses its clockwise direction aboutevery pitch distance (which is 0.3 inch). The turbulator substantiallyimproves heat exchanger performance. Tests by Applicants have shown thatthe addition of the turbulator reduces the required water flow by afactor of roughly 5 to maintain comparable gas temperature conditions.

Flow Path and Acoustic Effects

In this preferred embodiment, gas flow into and out of the dischargeregion has been greatly improved over prior art laser chambers. Theregion upstream of the discharge and adjacent to the exit of the crossflow fan is shaped to form a smooth transition from a large crosssection to the small cross section of the discharge.

The cross section of the region directly downstream of the dischargeincreases smoothly for the small value of the discharge to a muchgreater value before the gas is forced to turn 90° into the heatexchangers. This arrangement minimizes the pressure drop and associatedturbulence caused by high velocity flow over sharp steps. Providing thissmooth gradually expanding flow path in the direction away from thelaser also reduces adverse acoustic effects resulting from acousticwaves from a pulse reflecting back to the discharge region at the timeof a subsequent pulse. Techniques for reducing these effects aredescribed in U.S. Pat. Nos. 6,212,211 and 6,317,447 both of which areincorporated herein by reference. The time required for an acoustic waveto return to the discharge region is dependent to a significant extent.The result is reflection from a particular surface could be a problemonly at a particular combination of repetition rate and gas temperature.If this reflecting surface cannot be easily eliminated an alternatesolution could be to avoid operation at the problemtemperature-repetition rate combination. One solution could be toprogram the laser controller to automatically change the gas temperatureas necessary to avoid operation at a problem combination.

Blower Motors and Large Blower

This first preferred embodiment of the present invention provides alarge tangential fan driven by dual motors for circulating the lasergas. This preferred arrangement as shown in FIG. 24 provides a gas flowbetween the electrode of 67 m/sec which is enough to clear a space ofabout 1.7 cm in the discharge region between 4,000 Hz pulses.

A cross section blade structure of the fan is shown as 64A in FIG. 4. Aprospective view is shown in FIG. 18A. The blade structure has a 5 inchdiameter and is machined out of a solid aluminum alloy 6061-T6 barstock. The individual blade in each section is slightly offset from theadjacent section as shown in FIG. 18A. The offset is preferably madenon-uniform so as to avoid any pressure wave front creation. As analternative, the individual blades can be slightly angled with respectto the blade axis (again to avoid creation of pressure wave fronts). Theblades also have sharp leading edges to reduce acoustic reflections fromthe edge of the blade facing the discharge region.

This embodiment as shown in FIG. 18 utilizes two 3 phase brushless DCmotors each with a magnetic rotor contained within a metallic pressurecup which separates the stator portion of the motors from the laser gasenvironment as described in U.S. Pat. No. 4,950,840. In this embodiment,the pressure cup is thin-walled nickel alloy 400, 0.016 inch thick whichfunctions as the laser gas barrier. The two motors 530 and 532 drive thesame shaft and are programmed to rotate in opposite directions. Bothmotors are sensorless motors (i.e., they operate without positionsensors). Right motor controller 534 which controls right motor 530functions as a master controller controlling slave motor controller 536via analog and digital signals to institute start/stop, current command,current feedback, etc. Communication with the laser controller 24A isvia a RS-232 serial port into master controller 534.

Purge System

This first embodiment of the present invention includes an ultra-pure N₂purge system which provides greatly improved performance andsubstantially increases component lifetime.

FIG. 19 is a block diagram showing important features of a firstpreferred embodiment the present invention. Five excimer lasercomponents which are purged by nitrogen gas in this embodiment of thepresent system are LNP 2P, high voltage components 4P mounted on laserchamber 6P, high voltage cable 8P connecting the high voltage components4P with upstream pulse power components 10P, output coupler 12P andwavemeter 14P. Each of the components 2P, 4P, 8P, 12P, and 14P arecontained in sealed containers or chambers each having only two ports anN₂ inlet port and an N₂ outlet port. An N₂ source 16P which typically isa large N₂ tank (typically maintained at liquid nitrogen temperatures)at a integrated circuit fabrication plant but may be a relatively smallbottle of N₂. N₂ source gas exits N₂ source 16P, passes into N₂ purgemodule 17P and through N₂ filter 18P to distribution panel 20Pcontaining flow control valves for controlling the N₂ flow to the purgedcomponents. With respect to each component the purge flow is directedback to the module 17P to a flow monitor unit 22P where the flowreturning from each of the purge units is monitored and in case the flowmonitored is less than a predetermined value an alarm (not shown) isactivated.

FIG. 19A is a line diagram showing specific components of this preferredembodiment including some additional N₂ features not specificallyrelated to the purge features of the present invention.

N₂ Filter

An important feature of the present invention is the inclusion of N₂filter 18. In the past, makers of excimer lasers for integrated circuitlithography have believed that a filter for N₂ purge gas was notnecessary since N₂ gas specification for commercially available N₂ isalmost always good enough so that gas meeting specifications is cleanenough. Applicants have discovered, however, that occasionally thesource gas may be out of specification or the N₂ lines leading to thepurge system may contain contamination. Also lines can becomecontaminated during maintenance or operation procedures. Applicants havedetermined that the cost of the filter is very good insurance against aneven low probability of contamination caused damage.

A preferred N₂ filter is Model 500K Inert Gas Purifier available fromAeronex, Inc. with offices in San Diego, Calif. This filter removes H₂O,O₂, CO, CO₂, H₂ and non-methane hydrocarbons to sub-parts-per-billionlevels. It removes 99.9999999 percent of all particulate 0.003 micronsor larger.

Flow Monitors

A flow monitor in unit 22 is provided for each of the five purgedcomponents. These are commercially available units having an alarmfeature for low flow.

Piping

Preferably all piping is comprised of stainless steel (316SST) withelectro polished interior. Certain types of plastic tubing, comprised ofPFA 400 or ultra-high purity Teflon, may be also used.

Recirculation and Clean Up

A portion or all of the purge gas could be recirculated as shown in FIG.19B. In this case, a blower and a water cooled heat exchanger is addedto the purge module. For example, purge flow from the optical componentscould be recirculated and purge flow from the electrical componentscould be exhausted or a portion of the combined flow could be exhausted.Also, an ozone clean-up element could be added to remove ozone from theenclosed beam path. This could include a filter made of one of severalmaterials reactive with O₃.

Improved Seals

Preferred techniques for enclosing the beam path are described in U.S.patent application Ser. No. 10/000,991, filed Nov. 14, 2001, entitled“Gas Discharge Laser With Improved Beam Path” which is incorporated byreference herein. FIGS. 19F1, 2, 3, 4 and 5 show easy sealing bellowsseal used to provide seals between the laser modules but allowing quickeasy decoupling of the modules to permit quick module replacement.

Easy Sealing Bellows Seal

Applicants have developed an easy sealing bellows seals to permit quicksealing of the beam path to a vacuum compatible seal when reinstallinglaser modules in the beam path. The reader should note that although theseals provide vacuum quality seals of the respective sealed portions ofthe beam path, the path is not operated as a vacuum but typically atpressures slightly in excess of atmospheric.

Fast sealing is important since there is a great need that these modulesbe replaceable within a few minutes. The basic design of the easysealing bellows seal is shown in FIGS. 8A-E. The easy sealing bellowsseals are a four part seal. These four paths are (1) bellows part 93Ashown in FIG. 8A, flange part 93B shown in FIGS. 8A and 8B metal c sealring 93C shown in FIG. 8A and a first compression ring clamp 93D shownin FIG. 8C. An alternate second compression ring clamp is shown in FIG.8E. An easy seal bellows is shown assembled in FIG. 8D. Two additionalmetal C-seals may be used to seal flange part 93B to a first laser part93E and to seal bellows portion 93A to a second laser part 93F. Theseadditional seals are placed in slots 102 and 104. Flange part 93B issealed against the first laser part with screws through counter sunkholes 106 and which are tightened with an allen wrench through holes108.

Flange part 93B comprises tapered flange 120. This flange has a 20°taper as shown in FIG. 8A. Flange 114 also has a 20° taper. Compressionclamp 93D is then opened up by unscrewing finger bolt 118 and placedaround tapered flanges 120 and 114. Compression clamp 93D has hingesection 122 and a bolt section 124. It has a tapered slotted innercircumference matching the type of flanges 114 and 120. The diameter ofthe slot with bolt 118 fully inserted is slightly smaller than thematching slanted surfaces of flanges 114 and 120 so that as bolt 118 istightened the two flanges are forced together compressing c seal 93Cbetween them to produce a vacuum compatible seal. Applicants havedetermined that 400 pounds of compression is preferred to assure thedesired vacuum seal. This requires a torque of about 40 inch-poundsapplied to the handle of bolt 118 of the first compression ring clamp.In this preferred embodiment, the handle is only 1-inch long so a speedwrench (or similar tool) would be needed by most technicians to providethe 40 inch pounds. If a two inch handle is provided the seal could bemade with finger force. The second compression ring clamp shown in FIG.8E forces the two tapered flanges together when curved lever arm 119 ispushed into position against the circumference of the ring. The clamp isopened by rotating the arm out from the ring circumference and then thetwo halves of the ring clamp can be separated. Applicants have estimatedthat a 40 pound force applied to the end of lever arm 119 results in acompressive force of about 400 pounds on C-seal 93C. This clamp designis based on the design of commercially available clamps known as “pullaction toggle clamps”.

Important advantages of this seal system are:

(1) The time to make the seal is insignificant (about 1 to 2 minutes);

(2) An excellent vacuum seal is produced;

(3) Substantial vibrational coupling between the chamber and opticalcomponents is avoided; and

(4) The seal is inexpensive compared to most other vacuum sealingtechniques.

The seal is made between flange part 93B and 93A with metal c seal 93Csandwiched in between the two parts as indicated in FIG. 8A usingcompression ring clamp 93D as shown in FIG. 8D. The metal seal fits intoslot 110. The seal in these embodiments are made slightly oval to fit inthe circular slot 110. The longer diameter of the c seal ring is 1.946inches and the shorter diameter is 1.84 inches. Spring force in the ovalshaped c seal ring produces forces against the edge of slot 110 whichprevents the c seal ring from falling out during assembly. Bellows part93A comprises circular ridge 112 which protects seal ring 93C from beingscratched by part 93B while the two parts are slid against each otherduring assembly.

Transverse Purge Gas Flow

Applicants have discovered through experiments with F2 lasers at highrepetition rates, such as 4000 Hz, several milli Joule per pulse beamwill suffer divergence and deflectance transients due to beaminteraction with the purge gas. Applicants have also determined thatthese effects can be minimized by producing purge flows transverse tothe beam path. Applicants have identified several techniques for doingthis. Four such techniques are shown in FIGS. 19C1, 19C2, 19C3, and19C4. FIGS. 19C1 and 2 show baffles in the beam path which encouragetransverse purge flow. FIG. 19C3 show a fan in the purge line forrecirculating the purge gas and in 19C4 the purge flow is directed bypurge nozzles transverse to the beam path.

Alignment Laser

Providing a confined vacuum type purge path complicates alignment of thelaser optics. In prior art systems, the purge path had to be broken toinsert a small visible light alignment laser. In preferred embodiments,a small visible light laser may be included as a permanent part of thebeam path which is very useful during maintenance operations.Preferably, the alignment laser is a helium-neon laser or small diodelaser mounted on the rear side of high reflectance mirror 10D which forthis design should be comprised of a CaF plate having a dielectricreflecting coating designed to reflect a very high portion of 157 nmultraviolet light but transmit a high portion of visible light. Thealignment laser can then be used for alignment of the entire beam paththrough the MO, the PA and the beam stretcher, all without breaking thepurge path.

Advantages Of the Vacuum Quality Beam Path

The vacuum quality purge system described herein represents a majorimprovement in long term excimer laser performance especially for F₂lasers. Contamination problems are basically eliminated which hasresulted in substantial increases in component lifetimes and beamquality. In addition, since leakage has been eliminated except throughoutlet ports the flow can be controlled to desired values which has theeffect of reducing N₂ requirements by about 50 percent.

Sealed Shutter Unit with Power Meter

This first preferred embodiment includes a sealed shutter unit 500 witha built in power meter as shown in FIGS. 20, 20A and 20B. With thisimportant improvement, the shutter has two functions, first, as ashutter to block the laser beam and, second, as a full beam power meterfor monitoring beam power whenever a measurement is needed.

FIG. 20 is a top view showing the main components of the shutter unit.These are shutter 502, beam dump 504 and power meter 506. The path ofthe laser output beam with the shutter in the closed position is shownat 510 in FIG. 20. The path with the beam open is shown at 512. Theshutter active surface of beam stop element 516 is at 45° with thedirection of the beam exiting the chamber and when the shutter is closedthe beam is both absorbed in the shutter surface and reflected to beamdump 504. Both the beam dump active surface and the shutter activesurface is chrome plated for high absorption of the laser beam. In thisembodiment, beam stop element 516 is mounted on flexible spring steelarm 518. The shutter is opened by applying a current to coil 514 asshown in FIG. 20B which pulls flexible arm 518 and beam stop element 516to the coil removing beam stop element 516 from the path of the outputlaser beam. The shutter is closed by stopping the current flow throughcoil 514 which permits permanent magnets 520 to pull beam stop element516 and flexible arm 518 back into the close position. In a preferredembodiment the current flow is carefully tailored to produce an easytransmit of the element and arm between the open and close positions.

Power meter 506 is operated in a similar fashion to place pyroelectricphoto detector in the path of the output laser beam as shown in FIGS. 20and 20A. In this case, coil 520 and magnets 522 pull detector unit 524and its flexible arm 526 into and out of the beam path for output powermeasurements. This power meter can operate with the shutter open andwith the shutter closed. Current to the coil is as with the shuttercontrolled to provide easy transit of unit 524 into and out of the beampath.

Weak Line-Strong Line Monitor

KrF and ArF lasers currently used for lithography light sources forintegrated circuit manufacture comprise complicated wavemeter modulesfor monitoring the center line wavelength and the bandwidth of the laserbeam. These monitors also provide feedback signals which are used tocontrol the center line wavelength.

Preferred embodiments the present invention comprise a weak line-strongline monitor to provide some of the functions of the wavemeters of theprior art. FIG. 25 is a drawing of such a monitor. A portion of theoutput beam 200 from the power amplifier is picked off by uncoated wedge202 and passes through attenuator 204 to strong line weak line separatoroptical unit 206. The separator unit comprises five 65 degree prismsarranged using well-known optical techniques to produce an 8.3milliradian separation of the two lines with no magnification and atransmission if 72% of the beam is directed through attentuator 210 anddiffuser 212 to photomultiplier tube 214 which monitors the intensity ofboth the weak line and the strong line. About 98 percent of the outputof separator 206 passes through wedge 208 to lens 216 which focuses theseparated beam at the location of knife edge 218. Knife edge 218 blocksthe strong line and allows the weak line to pass and pass throughdiffuser 220 to be detected by PMT 222. The weak line-strong line ratiois then determined by a computer based on signals from the two PMTdetectors.

Improved Polarization with Chamber Windows Angle Greater than Brewster

Prior art chamber windows are often placed at Brewster's angle whichresults in about 100% transmission of the polarization direction atabout 58 percent in the p-polarization direction. In other prior artdesigns, the windows are placed at bout 45 degrees in which casetransmission of the s-polarization is a little less and the p a littlemore than the above values.

For F₂ lasers, Applicants have determined that with the chamber windowsat somewhat less than Brewster's, there is substantial competition inthe gain region between the s and p polarization. This is because theretypically is very high gain in the discharge region of the F₂ laserscompared to prior art KrF and ArF lasers. This competition is notdesirable because in most applications light at s-polarization is notuseful and is typically lost as undesirable heat. Thus, there is a needto minimize the amount of s-polarization produced in the laser.

A preferred technique which can be relatively easily accomplished is toincrease the angle of incidence of the chamber windows substantiallyabove the Brewster's angle. For example, at Brewster's angle for a 157nm F₂ beam about 100% of the p-polarization is transmitted and about 83%of the s-polarization is transmitted. If we increase the angle ofincident to 64° the p-polarization transmission is reduced to about 99%but only 76% of the s-polarization is transmitted. Since in the masteroscillator the light output from the gain region has made about twopasses through each of the two windows (for 4 window passes and 2surfaces for each window) the ratio of the two polarizations in theoutput beam (assuming 64° window angles) is:$\text{Polarization Extinction Ratio} = {\frac{1}{1 + \left( \frac{I_{s}}{I_{p}} \right)^{8}} = {\frac{1}{1 + \left( \frac{0.76}{0.99} \right)^{8}} = {89\%}}}$

Applicants tests with 47° windows about 72% of the light isp-polarization and 28% at s-polarization.

By changing the angle of the windows to 64° and adding an additionalwindow at 64° in front of the high reflection mirror 10D in FIG. 1 thepercent of s-polarization in the output beam is reduced to about 4% andthe remaining 96% of the light is at p-polarization.

Various modifications may be made to the present invention withoutaltering its scope. Those skilled in the art will recognize many otherpossible variations. For example, the pulse power circuit could be acommon circuit up to the output of pulse transformer 56 as shown in FIG.5. In some cases, customer specifications could permit a weak line aslarge as about 0.5 percent of the strong line. Applicants preferred gasspecifications for the MO is a fluorine partial pressure of 0.2 kPa anda total gas pressure of 200 kPa but as the FIG. 24 graphs demonstratemany other combinations give good results. This approach provides for afurther reduction in jitter as explained in U.S. patent application Ser.No. 09/848,043 which is incorporated herein by reference. FIG. 3B ofthat patent application showing the input and output to the pulsetransformer is included herein as FIG. 13 for the convenience of thereader. Other heat exchanger designs should be obvious modifications tothe one configuration shown herein. For example, all four units could becombined into a single unit. There could be significant advantages tousing much larger fins on the heat exchanger to moderate the effects ofrapid changes in gas temperature which occurs as a result of burst modeoperation of the laser. The reader should understand that at extremelyhigh pulse rates the feedback control on pulse energy does notnecessarily have to be fast enough to control the pulse energy of aparticular pulse using the immediately preceding pulse. For example,control techniques could be provided where measured pulse energy for aparticular pulse is used in the control of the second or third followingpulse. Many other layout configurations other than the one shown in FIG.1 could be used. For example, the chambers could be mounted side-by-sideor with the PA on the bottom. Also, the second laser unit could beconfigured as a slave oscillator by including an output coupler such asa partially reflecting mirror. Other variations are possible. Fans otherthan the tangential fans could be used. This may be required atrepetition rates much greater than 4 kHz. The fans and the heatexchanger could be located outside the discharge chambers. Pulse timingtechniques described in U.S. patent application Ser. No. 09/837,035(incorporated by reference herein) could also be utilized. Lineselection techniques other than the five-prism design described abovecould be used. For example, the strong line could be selected using 3, 4or 6 prisms and applying the design techniques discussed above.Measurement of the bandwidth with precision may be desired. This couldbe done with the use of an etalon having a smaller free spectral rangethan the etalons described above. Other techniques well known could beadapted for use to precisely measure the bandwidth. Accordingly, theabove disclosure is not intended to be limiting and the scope of theinvention should be determined by the appended claims and their legalequivalents.

We claim:
 1. A very narrow band two chamber high repetition rate F₂ gasdischarge laser system comprising: A) a first laser unit comprising: 1)a first discharge chamber containing; a) a first laser gas comprisingfluorine gas defining a fluorine partial pressure and at least onebuffer gas wherein said laser gas defines a to gas pressure b) a firstpair of elongated spaced apart electrodes defining a first dischargeregion, 2) a first fan for producing sufficient gas velocities of saidfirst laser gas in said first discharge region to clear from said firstdischarge region, following each pulse, substantially all dischargeproduced ions prior to a next pulse when operating at a repetition ratein the range of 2,000 pulses per second or greater, 3) a first heatexchanger system capable of removing at least 16 kw of heat energy fromsaid first laser gas, B) a second laser unit comprising: 1) a seconddischarge chamber containing: a) a second laser gas, b) a second pair ofelongated spaced apart electrodes defining a second discharge region 2)a second fan for producing sufficient gas velocities of said secondlaser gas in said second discharge region to clear from said seconddischarge region, following each pulse, substantially all dischargeproduced ions prior to a next pulse when operating at a repetition ratein the range of 4,000 pulses per second or greater, 3) a second heatexchanger system capable of removing at least 16 kw of heat energy fromsaid second laser gas, C) a pulse power system configured to provideelectrical pulses to said first pair of electrodes and to said secondpair of electrodes sufficient to produce laser pulses at rates of about4,000 pulses per second with precisely controlled pulse energies inexcess of about 5 mJ, D) a laser beam measurement and control system formeasuring pulse energy of laser system output pulses produced by saidlaser system and controlling said gas discharge laser output pulses in afeedback control arrangement, and wherein output laser beams from saidfirst laser unit are utilized as a seed beam for seeding said secondlaser unit and wherein said fluorine partial pressure in said firstchamber and said total gas pressure is said first chamber are low enoughthat spectral intensity of the laser system output pulses at about157.52 nm is less than 0.5% of spectral intensity of the laser systemoutput pulses at 157.63 nm.
 2. A laser system as in claim 1 wherein saidfirst laser unit is configured as a master oscillator and said secondlaser unit is configured as a power amplifier.
 3. A laser system as inclaim 2 wherein said first laser gas comprises fluorine and helium.
 4. Alaser system as in claim 2 wherein said first laser gas comprisesfluorine and neon.
 5. A laser system as in claim 2 wherein said firstand second laser gas comprises fluorine and a buffer gas chosen from agroup consisting of neon, helium or a mixture of neon and helium.
 6. Alaser system as in claim 1 wherein said spectral intensity of the lasersystem output pulses at about 152.52 is less than 0.01% of the spectralintensity of the laser system output pulses at 157.63 nm.
 7. A lasersystem as in claim 1 wherein said fluorine partial pressure is less than0.3 kPa and the total laser gas pressure is less than 270 kPa.
 8. Alaser system as in claim 1 wherein said fluorine partial pressure isless than about 0.2 kPa and the total laser gas pressure is less thanabout 200 kPa.
 9. A laser system as in claim 1 wherein said fluorinepartial pressure is about 0.2 kPa and the total laser gas pressure isabout 200 kPa.
 10. A laser system as m claim 1 wherein said fluorinepartial pressure is between 0.05 kPa and 0.3 kPa.
 11. A laser system asin claim 1 wherein said total laser gas pressure is between 100 kPa and270 kPa.
 12. A laser system as in claim 1 and further comprising a weakline-strong line monitor for monitoring spectral intensities for thepurpose of determining spectral intensity of the laser system outputbeam at about 157.63 nm relative to intensity of all spectral componentsthe laser system output beam.
 13. A laser system as in claim 1 andfurther comprising a vertically mounted optical table with said firstand second discharge chambers mounted on said vertical optical table.14. A laser as in claim 1 and wherein first and second said chamberseach comprises a vane structure downstream of said discharge region fornormalizing gas velocity downstream of said discharge region.
 15. Alaser as in claim 1 wherein said first fan and said second fan each aretangential fans and each comprises a shaft driven by two brushless DCmotors.
 16. A laser as in claim 15 wherein said motors are water-cooledmotors.
 17. A laser as in claim 15 wherein each of said motors comprisea stator and each of said motors comprise a magnetic rotor contained ina pressure cup separating a said stator from said laser gas.
 18. A laseras in claim 1 wherein said first and second fans are each tangentialfans each comprising a blade structure machined from said aluminumstock.
 19. A laser as in claim 15 wherein said blade structure has anoutside diameter of about five inches.
 20. A laser as in claim 19wherein said blade structures comprise blade elements having sharpleading edges.
 21. A laser as in claim 15 wherein said motors aresensorless motors and further comprising a master motor controller forcontrolling one of said motors and a slave motor controller forcontrolling the other motor.
 22. A laser as in claim 15 wherein each ofsaid tangential fans comprise blades which are angled with respect tosaid shaft.
 23. A laser as in claim 1 wherein each finned heat exchangersystem is water cooled.
 24. A laser as in claim 23 wherein each heatexchanger system comprises at least four separate water cooled heatexchangers.
 25. A laser as in claim 23 wherein each heat exchangersystem comprises at least one heat exchanger having a tubular water flowpassage wherein at least one turbulator is located in said path.
 26. Alaser as in claim 25 wherein each of said four heat exchangers comprisea tubular water flow passage containing a turbulator.
 27. A laser as inclaim 26 wherein said high voltage is isolated from ground using aninductor through which cooling water flows.
 28. A laser as in claim 1wherein said pulse power system comprises a first charging capacitorbank and a first pulse compression circuit for providing electricalpulses to said first pair of electrodes and a second charging capacitorbank and a second pulse compression circuit for providing electricalpulses to said second pair of electrodes and a resonant charging systemto charge in parallel said first and second charging capacitor banks toa precisely controlled voltage.
 29. A laser as in claim 28 wherein saidresonance charging system comprises a De-Qing circuit.
 30. A laser as inclaim 28 wherein said resonance charging system comprises a bleedcircuit.
 31. A laser as in claim 28 wherein said resonant chargingsystem comprises a De-Qing circuit and a bleed circuit.
 32. A laser asin claim 1 wherein said pulse power system comprises a charging systemcomprised of at least two power supplies arranged in parallel.
 33. Alaser as in claim 1 and further comprising a line narrowing unit islocated downstream of said master oscillator and upstream of said poweramplifier.
 34. A laser as in claim 1 wherein said first dischargechamber and said second discharge chamber comprise chamber windowspositioned so that the incident angles of laser beams on said windowsare all greater than Brewster' angle.
 35. A laser as in claim 1 andfurther comprising a beam steering means for steering laser beamsproduced in said first laser unit.
 36. A laser as in claim 35 whereinsaid steering means comprises a means for pivoting an optical element.37. A laser as in claim 35 wherein said beam steering means comprises ameans for adjusting the pressure in said line selection unit.
 38. Alaser as in claim 1 wherein said laser system comprises prism outputcoupler in part defining a resonant cavity for said first laser unit,said prism output coupler comprises two surfaces, a first surfaceoriented at a low loss angle for p-polarization and a second surfacelocated orthogonal to laser beams from said first laser unit.
 39. Alaser as in claim 1 and further comprising a beam enclosure systemcomprising: A) at least one beam seal said beam seals comprising a metalbellows, and B) a purge means for purging said beam enclosure with apurge gas.
 40. A laser as in claim 39 wherein said beam enclosure meanscomprise a flow directing means for producing purge flow transverse tolaser beams produced in said second laser unit.
 41. A laser as in claim39 wherein said at least one beam seal is configured to permit easyreplacement of said laser chamber.
 42. A laser as in claim 39 whereinsaid at least one beam seal contains no elastomer, provide vibrationisolation from said chamber, provide be train isolation from atmosphericgases and permit unrestricted replacement of said laser chamber withoutdisturbance of said line selection unit.
 43. A laser as in claim 39wherein said at least one beam seal is vacuum compatible.
 44. A laser asin claim 43 wherein said at least one beam seal is a plurality of beamseals and said plurality of said seals are easy sealing bellows sealsconfigured for easy removal by hand.
 45. A laser as in claim 1 whereinsaid measurement and control system comprises a primary beam splitterfor splitting off a small percentage of output pulses from said secondlaser unit and an optical means for directing a portion of said smallpercentage to said pulse energy detector and an isolation means forisolating a volume bounded at least in part by said primary beamsplitter and a window of said pulse energy detector from other portionsof said measurement and control system to define an isolated region. 46.A laser as in claim 45 and further comprising a purge means for purgingsaid isolated region with a purge gas.
 47. A laser system as in claim 1wherein said system is configured to operate either of a KrF lasersystem, an ArF laser system or an F₂ laser system with minormodifications.
 48. A laser system as in claim 1 wherein said pulse powersystem comprises a master oscillator charging capacitor bank and a poweramplifier charging capacitor bank and a resonant charger configured tocharge both charging capacitor banks in parallel.
 49. A laser as inclaim 48 wherein said pulse power system comprises a power supplyconfigured to furnish at least 2000V supply to said resonant charges.50. A laser as in claim 1 and further comprising a gas control systemfor controlling F₂ concentrations in said first laser gas to controlmaster oscillator beam parameters.
 51. A laser as in claim 1 and furthercomprising a gas control system for controlling laser gas pressure insaid first laser gas to control master oscillator beam parameters.
 52. Alaser as in claim 2 and further comprising a discharge timing controllerfor triggering discharges in said power amplifier to occur between 20and 60 ns after discharges in said master oscillator.
 53. A laser as inclaim 2 and further comprising a discharge controller programmed tocause in some circumstances discharges so timed to avoid any significantoutput pulse energy.
 54. A laser as in claim 53 wherein said controlledin said some circumstances is programmed to cause discharge in saidpower amplifier at least 20 ns prior to discharge in said masteroscillator.
 55. A laser as in claim 1 and further comprising a pulsemultiplier unit for increasing duration of output pulses from saidlaser.
 56. A laser as in claim 55 wherein pulse multiplier unit isarranged to receive said output pulse laser beam and to multiply thenumber of pulses per second by at least a factor of two so as to producea single multiplier output pulse beam comprised of a larger number ofpulses with substantially reduced intensity values compared with thelaser output pulses, and pulse multiplier unit comprising: (1) a firstbeam splitter arranged to separate a portion of said output beam, theseparated portion defining a delayed portion, and the output beamdefining a beam size and angular spread at said first beam splitter; (2)a first delay path originating and terminating at said first beamsplitter said first delay path comprising at least two focusing mirrors,said mirrors being arranged to focus said delayed portion at a focalpoint within said first delay path and to return said delayed portion tosaid first beam splitter with a beam size and angular spread equal to orapproximately equal to the beam size and angular spread of the outputbeam at said first beam splitter.
 57. A laser system as in claim 56wherein said at least two focusing mirrors are spherical mirrors.
 58. Alaser system as in claim 56 and further comprising a second delay pathcomprising at least two spherical mirrors.
 59. A laser system as inclaim 56 wherein said first delay path comprises four focusing mirrors.60. A laser system as in claim 59 and further comprising said seconddelay path created by a second beam splitter located in said first delaypath.
 61. A laser as in claim 56 wherein said first delay path comprisesa second beam splitter and further comprising a second delay pathcomprising at least two focusing mirrors, said mirrors being arranged tofocus said delayed portion at a focal point within said first delay pathand to return said delayed portion to said first beam splitter with abeam size and angular spread equal to or approximately equal to the beamsize and angular spread of the output beam at said first beam splitter.62. A laser as in claim 56 wherein said first beam splitter isconfigured to direct a laser beam in at least two directions utilizingoptical property of frustrated internal reflection.
 63. A laser as inclaim 56 wherein said first beam splitter is comprised of twotransparent optical elements each element having a flat surface, saidoptical elements being positioned with said surfaces parallel to eachother and separated by less than 200 nm.
 64. A laser as in claim 56wherein said first beam splitter is an uncoated optical element orientedat an angle with said output laser beam so as to achieve a desiredreflection-transmission ratio.